LONDON (AFP) - Thousands of people demonstrated across the world Saturday against Israel's airstrikes on Gaza, calling for an immediate halt to the military offensive. The biggest rallies were in London, where police said up to 12,000 people took part, and Paris where an estimated 25,000 took to the streets. Thousands of people also voiced their opposition to Israel's military action in demonstrations in Madrid, Amsterdam, Stockholm and the Austrian city of Salzburg. In London, many demonstrators carried Palestinian flags and chanted "Free, free Palestine" and "Israel terrorists" as they filed along the River Thames before gathering in Trafalgar Square, an AFP reporter saw. The march, which organisers Stop the War Coalition claimed had attracted 50,000 people, was led by singer Annie Lennox and veteran left-wing politician Tony Benn. British opposition lawmaker Sarah Teather said Israel's military response to the firing of Hamas rockets had been "disproportionate". Elsewhere in Britain, 2,000 people demonstrated in Manchester in northwest England and 500 braved the cold in the Scottish capital Edinburgh. In Paris, where organisers said 25,000 turned out for a march led by Communist and left-wing politicians, the crowd chanted, "We are all Palestinians" and "Israel killers". Two people were arrested as more than 1,000 marched through the Dutch city of Amsterdam condemning the Israeli airstrikes on Gaza and calling for a boycott of Israeli goods, police said. Hundreds protested in Madrid, carrying signs saying, "This is not a war but a genocide". More than 2,000 people also demonstrated in the Austrian city of Salzburg. Meanwhile, tens of thousands of Israeli Arabs demonstrated on Saturday in the northern town of Sakhnin over Israel's week-old offensive against the Hamas-controlled Gaza Strip. Organisers estimated that nearly 100,000 people attended the protest rally, which stretched through the city streets. Crowds waving Palestinian flags and brandishing pro-Palestinian placards chanted "Gaza will not surrender to the tanks and bulldozers" and "Don't fear, Gaza, we are with you" Thousands of police deployed on the outskirts of Sakhnin and across northern Israel following a number of violent protests against the Gaza operation in recent days. Meanwhile, hundreds of Afghans marched in new demonstrations Saturday against Israel's deadly strikes on the Gaza Strip, with protesters in the capital also demanding that US troops pull out of Afghanistan. The rallies followed demonstrations by thousands of Afghans on Friday, at which protesters demanded an end to the Israeli attacks that began on December 27 and have so far killed more than 430 Palestinians. In Kabul, up to 600 demonstrators marched in the city centre, holding banners that read "Israel commits crimes, America supports," and shouting "Death to Israel" and "Death to America." Meanwhile, British Prime Minister Gordon Brown urged his Israeli counterpart Ehud Olmert to halt military action in Gaza in a phone call on Saturday, Brown's office said. "The prime minister has spoken again today to Prime Minister Olmert, and is pressing hard for an immediate ceasefire," a statement said. "Rocket attacks from Hamas must stop, and we have called for a halt to Israeli military action in Gaza. "Too many have died, and we need space to get humanitarian supplies to those who need them. "As the prime minister has made clear, moderation must prevail." Brown also said action was "necessary" to re-open Egypt's border with Gaza, "but in a way that does not undermine Israel's security". Britain has refused to condemn Israel's campaign of airstrikes in response to the firing of Palestinian rockets into Israeli territory. Meanwhile, Jordan's King Abdullah II told Middle East peace Quartet envoy Tony Blair on Saturday that the world's "silence" on the humanitarian situation in the Gaza Strip is unacceptable, the palace said. "The humanitarian situation in Gaza has deteriorated in a way that silence has become unacceptable," a palace statement quoted the king as telling Blair over the telephone. "The international community should help end this tragedy and press Israel to stop its aggression and open border crossings to allow aid and evacuate the wounded," the king said, according to the statement. The Quartet includes the United States, the United Nations, the European Union, and Russia. The king and Blair discussed "international efforts to halt Israel's military operations in Gaza," the statement said. Jordan plans to build a field hospital in Gaza and send daily aid shipments to densely populated and impoverished territory. On Thursday it evacuated eight Gazans from the Egyptian's city of El-Arish near Gaza to treat them in a military hospital in Amman.