TEHRAN/GENEVA  - Iranians rallied nationwide on Friday in a show of support for Palestinians as archfoe Israel pursued its deadly campaign against the Gaza Strip.

Demonstrations were held in Tehran and more than 700 towns and cities across the country on the last day of prayer and rest of the Muslim holy fasting month of Ramazan, state television reported.

In the capital, footage showed demonstrators, carrying placards proclaiming "Death to Israel" and "Death to America", converging from nine different points on Tehran University in the city centre.

Iran holds Quds Day (Jerusalem Day) rallies in support of the Palestinians every year on the last Friday of Ramazan, but this year's demonstrations came on the 18th day of Israel's deadly campaign against rocket-firing fighters in Gaza.

More than 800 Palestinian civilians have been killed in the assault on Gaza and the Hamas movement that dominates it and has long been supported by Iran.

Rockets and mortar rounds fired into Israel have killed three civilians - two Israelis and a Thai farm worker - and fighting in and around Gaza has killed 32 Israeli soldiers.

"The Islamic world must in unison declare this day one of anger, hatred, unity and resistance against Israel," President Hassan Rouhani said at the Tehran demonstration.

Rouhani, who has overseen a fledgling rapprochement with the West condemned, "those who stay silent in the face of the Zionist regime's crimes". "The world demands an end to the Gaza blockade, opening the Rafah crossing and halting attacks on Gaza so its people can live normally," he said.

He was referring to the demands of Hamas for any truce with Israel to end the deadly violence. The Rafah border crossing between Gaza and Egypt is the territory's only one not controlled by Israel.

General Hossein Salami, second in command of Iran's elite Revolutionary Guards, struck a defiant tone in a speech broadcast on state television.

"Now no place in the occupied territories is safe for the Zionists. The missiles of Palestinian fighters have a range well beyond what the Zionists believe," he said. "We will continue house by house and avenge the blood of martyrs shed in Palestine." Salami's sentiments were echoed by demonstrators. "My message to the Palestinians is this: continue the struggle until your last drop of blood. Islamic countries, especially Iran, are behind you to save you," a young protester called Hassan told AFP.

Iran does not recognise Israel's existence, and supports Palestinian Islamist groups that fight it. On Thursday, the speaker of Iran's parliament, Ali Larijani, told state television's Arabic service that Tehran had provided Hamas with the technology it has used to rain down rockets on Israel. "Today, the fighters in Gaza have good capabilities and can meet their own needs for weapons," he said. "But once upon a time, they needed the arms manufacture know-how and we gave it to them."

Supreme leader Ayatollah Ali Khamenei on Wednesday called on the Palestinians to keep fighting Israel and to expand their resistance from Gaza to the occupied West Bank. During the last major conflict in and around Gaza in November 2012, Larijani said Iran was "proud" to have provided "both financial and military support" to Hamas.

Israel accused Iran of supplying Gaza fighters with its Fajr-5 missile, which has a range of 75 kilometres (45 miles), for use during that conflict. But the commander of Iran's Revolutionary Guards, General Mohammad Ali Jafari, said at the time it was not the missiles that had been supplied but their technology.

Meanwhile, the World Health Organization urged on Friday the creation of a humanitarian corridor in the besieged Gaza Strip to allow for the evacuation of the wounded.

"WHO is calling for the creation of a humanitarian corridor ... to reach various crossings and to help in the evacuation of the large number of injured people," said Paul Garwood, a spokesman for the UN health agency.

"There's a daily increase in casualty figures," he told reporters.

Garwood said WHO representatives had been in contact in recent days with Israel as well as Egypt to discuss the possibility of setting up a corridor.

He said that as of Thursday, the total number of people injured in Gaza since the start of the Israeli offensive on July 8 stood at 5,118, among them 1,561 children, 1,700 women and 203 elderly.

"The huge strains on the facilities inside Gaza, coupled with the challenges to get replenishments to those facilities, and the increasing insecurity on a daily basis is increasing the number of people who are getting injured, and they need better medical care," he added.

A further concern is attacks on health facilities and medical staff, with hospitals, clinics and ambulances being damaged or destroyed. "This underscores the need for health facilities, patients and staff to be protected. It's a humanitarian right," Garwood said.

On Friday, the Palestinian death toll since Israel launched its offensive reached 815, the overwhelming majority of them civilians. Nearly 100 Palestinians were killed on Thursday, one of the bloodiest days of the conflict.

Rockets and mortar rounds fired into Israel by Palestinian fighters have killed three civilians - two Israelis and a Thai farm worker - and fighting in and around Gaza has killed 33 Israeli soldiers.

The conflict has also displaced tens of thousands of people in Gaza, many of whom have sought refuge at UN shelters. Turkish Foreign Minister Ahmet Davutoglu cancelled a scheduled trip to France and flew to Qatar on Friday to help efforts for a ceasefire between Israel and Hamas, a Turkish official said.

The decision comes after Davutoglu held a "teleconference with his counterparts from the United States and Qatar Thursday night and also spoke separately with the Palestinian groups," the official told AFP on condition of anonymity. "The minister made an abrupt decision to fly to Qatar to contribute to efforts for an immediate ceasefire."

US Secretary of State John Kerry on Thursday reached out to Hamas allies Turkey and Qatar to push for a ceasefire. Hamas leader Khaled Meshaal is based in Doha.

Turkey, a staunch critic of the Israeli assaults in Gaza, is pressing for involvement of Hamas in any negotiations for a ceasefire.

Turkish Prime Minister Recep Tayyip Erdogan, who presents himself as a rare global Muslim leader who stands up for Palestinian rights, has accused Israel of carrying out a "genocide" in Gaza.

His rhetoric sparked condemnation from Israel and severe criticism from Turkey's NATO ally the United States.

Meanwhile, French police said Friday that a protest planned for the weekend in Paris against Israel's Gaza offensive has been banned, following several similar rallies that turned violent.

Organisers of the rally scheduled for Saturday immediately lodged a legal challenge, their lawyer, Hosni Maati, said. Three pro-Palestinian rallies, two of them which had been banned but took place anyway, degenerated earlier this month, with some protesters looting, hurling anti-Semitic slogans and clashing with police.