Tens of thousands protested in London Saturday afternoon against Israel’s military operations in Gaza, denouncing Israel as a terrorist state and castigating British Prime Minister David Cameron for backing Israel’s right to self-defense against Hamas rocket fire.

Led by speakers on a podium, protesters holding placards and banners chanted pro-Palestinian and anti-Israel slogans.

At one point, a woman on the podium shouted “from the river to the sea” — a call for the elimination of Israel — and protesters responded by yelling “Palestine will be free.”

The crowd also directed shouts of “Shame on you” at Cameron, who publicly backed Israel’s right to defend itself against Hamas rocket fire aimed at the Israeli civilian population.

The Palestine Solidarity Campaign, which organized the demonstration, said that since the protest began at 12 p.m. (GMT), tens of thousands of people had gathered, more than the 20,000 anticipated. The demonstration began outside Downing Street and was to march to the Israeli embassy.

“London has already shown its outrage at Israel’s attacks on the mostly refugee population of Gaza, with people turning out in their thousands last week,” PSC Director Sarah Colborne said in a press release on the organization’s website. “Today’s national demonstration will give people from across the country the chance to say enough is enough, Israel’s siege of Gaza and its occupation of Palestinian land has to end now.”

Since Israel began Operation Protective Edge 12 days ago, 337 people have died in Gaza. Hamas has fired around 1,400 rockets into Israeli territory, aimed mostly at southern cities, but also frequently reaching the population centers of the Tel Aviv metropolitan area and Jerusalem. Several Hamas rockets have also struck almost as far north as Haifa. Two Israeli civilians have died in rocket attacks, including a Bedouin man near Dimona Saturday.

Saturday’s demonstration in London came on the heels of numerous protests Thursday and Friday worldwide, including Cairo, Istanbul, Cape Town, Berlin, New York and Washington. Many of the protests have turned violent.

In Istanbul, police warded off hundreds of rioters who attempted on Thursday to storm the Israeli embassy building. Demonstrators in Ankara and Istanbul also hurled stones at several compounds where Israeli officials reside. Calls for the destruction of the Jewish state were heard in both Turkish cities. Police responded by firing tear gas canisters and water cannons at the crowds. Israel decided Friday to pull some of its diplomatic staff out of Turkey in the wake of the protests.

Adiv Sterman, AP and JTA contributed to this report.