RÖSZKE, Hungary/washington - Hungarian police on Wednesday fired teargas and water cannon at several hundred migrants protesting at being unable to cross the border from Serbia, AFP reporters said.

"The crowd on the Serbian side became aggressive and threw stones, bottles and sticks at police on the Hungarian side," a police statement said.

A video showed the migrants opening a gate at the border before retreating. Police said they had called in reinforcements to handle the crowd, noting that the barrier was "replaced by several ranks of police". "The police is protecting the border of Hungary and the EU while respecting the law and the principle of proportionality," it added.

An AFP reporter on the Serbian side said around 300 out of a group of 500 took part in the protest and confirmed that some were hurling objects at the police. Police fired at least 20 tear gas grenades as the crowd chanted slogans in Arabic with their fists in the air.

Children were crying from the effects of the tear gas, which causes respiratory problems as well as eye irritation. Several ambulances arrived at the scene.

There was also two helicopters flying overhead, one from the army and the other from the police. The incident happened at "Roszke II", an official border crossing point between Serbia and Hungary.

The clashes took place after Hungary sealed its southern border with Serbia, cutting off one of the biggest entry points for migrants into the European Union this year.

Meanwhile, United Nations Secretary-General Ban Ki-moon Wednesday expressed shock and alarm at the treatment of refugees on the border of Hungary and Serbia, adding that they should be treated with dignity and their human rights respected.

"I was shocked to see how these refugees and migrants were treated, it's not acceptable," he told a press conference as the 7oth session of the U.N. General Assembly got underway. "All the countries have their domestic problems, but since they are the people fleeing the wars and persecutions, then we must show our compassionate leadership.

"They must be treated with human dignity and human rights. "That's my consistent message to European and Asian leaders, wherever this migration and refugees are coming." Noting that brutal conflicts, breakdowns in basic governance, economic despair and other factors have generated displacements of people not seen since the Second World War, the secretary-general said, “Men, women and children fleeing war and persecution deserve real support, including asylum.”

“I ask those standing in the way of the rights of refugees to stand in their shoes. People facing barrel bombs and brutality in their country will continue to seek life in another. People with few prospects at home will continue to seek opportunity elsewhere. This is natural. It is what any of us would do for ourselves and for our children,” he pointed out. Ban saluted leaders and citizens in several countries, including Germany, Sweden and Austria, for opening doors and showing solidarity.

He also voiced appreciation for the financial generosity of many countries in addressing the humanitarian consequences, in particular the United Kingdom and Kuwait. The UN chief will convene a high-level meeting on 30 September to mobilize “a humane, effective and rights-based response” to the refugee crisis. “I urge all States to shoulder their responsibilities and live up to their legal obligations.” Replying to a question, Ban said after eight male secretaries general, it was time for a qualified woman to head the organization.

Serbian police could be seen holding handkerchiefs over their mouths because of the tear gas.

Speaking to the M1 TV news channel, Hungarian Foreign Minister Peter Szijjarto said he had asked Belgrade to have the Serbian police take action to quell the disturbances.

Pressure is building for a special European Union summit to come up with solutions to the crisis, with the bloc bitterly split and its vaunted passport-free Schengen zone in jeopardy, as Germany boosted controls along parts of its frontier with France.

A small group of women and children entered EU member Croatia early on Wednesday, followed shortly afterwards by around 300 mostly Syrians and Afghans in the first of what could be a new surge of migrants seeking to reach the bloc by circumventing the razor-wire barrier erected by Hungary.

Croatia's Prime Minister Zoran Milanovic - who has denounced blasted Hungary's border fence as "unacceptable" - said migrants would be given free passage through the country, allowing them to push on towards Slovenia, Austria and Hungary's fenceless southwestern frontier. "We are ready to accept and direct those people, their religion and colour of skin is completely irrelevant, to where they apparently wish to go - Germany and Scandinavia," Milanovic told lawmakers.

AFP correspondents saw others heading towards the Croatian frontier - which is still peppered with minefields from the Balkan wars of the 1990s - after turning up in buses and taxis at the Serbian border town of Sid.

"We heard that Hungary was closed so the police told us we should come this way," Amadou, 35, from Mauritania in western Africa, told AFP. Hundreds more people were trapped Tuesday behind the barrier erected by Hungary along its border with Serbia in an attempt to halt the migrant tide.

Hungary's conservative Prime Minister Viktor Orban is planning a similar structure along the frontier with Romania, prompting outrage from its southern neighbour. But Budapest insisted the fence was working as it made its first arrests under tough new laws which punish "illegal border-crossing" with prison terms of up to three years.

The controversial measures are part of Orban's strategy to stem the flow of migrants trudging up through the western Balkans, most headed to Germany and Sweden.

The apparent success of the fence in deflecting the flow sparked fears in Serbia that it would be swamped by an unmanageable number of migrants.

Speaking to AFP, Serbia's minister for refugees, Aleksandar Vulin, called on Hungary to reopen its border "at least for women and children" where around a hundred people were waiting in vain to cross.

Hundreds more were stranded in the Turkish border city of Edirne after police stopped around 1,000 refugees from crossing into Greece to continue their journey towards Europe.

And huge crowds were camped out at Istanbul's main bus station for a second night running, after being refused tickets to Edirne.

"They cannot stay here. Maybe we will allow them to stay two or three days but then they have to leave," Edirne governor Dursun Ali Sahin told Turkey's NTV channel, a day after police surrounded its bus station to prevent the mostly Syrian refugees travelling west towards Greece and Bulgaria.

Hungary's hardline anti-migrant stance has been sharply criticised, with the UN refugee agency saying it could be in violation of the 1951 Refugee Convention.

The Council of Europe also said it was "concerned" about its draconian new border law and would press Orban for an explanation.

Romania, which is a member of the EU but not of the passport-free Schengen zone, criticised Hungary's planned border fence as "out of step with the spirit of Europe".

The Hungarian move came as others states moved to reintroduce border controls across Europe, which is grappling with its biggest migratory wave since the end of World War II.

Germany, Austria and Slovakia have introduced identity checks on parts of their borders, and Poland and the Netherlands are considering whether to follow suit.

The measures have caused tailbacks at road crossing points and stoked concern among German road hauliers, who point to rising costs.

Politically, though, the big concern is for the future of the 20-year-old Schengen agreement, considered by many EU supporters as important as the euro.

By doing away with border checks and reducing bureaucracy, it provides a powerful economic boost and enhances the notion of a common European identity, they say.

Berlin's decision Wednesday to extend greater passport controls to its border with France - Schengen's other principal architect - seemed to deal the accords another blow.

German Chancellor Angela Merkel and her Austrian counterpart Werner Faymann on Tuesday called for a special EU summit next week to debate the crisis.

"Time is running out," Merkel warned, urging an end to the squabbling that has flared since eastern members flatly refused to accept EU-set quotas for taking in tens of thousands of refugees.

European President Donald Tusk has said he will announce a decision about the summit on Thursday, with EU interior ministers to meet again next Tuesday in a fresh bid to resolve the quota wrangle.