Thousands of protesters gathered in London on Saturday to call for an end to Israeli military action in Gaza and "justice and freedom" for Palestine. Up to 15,000 people were expected to take part in the march, following a route from Downing Street to the Israeli embassy in Kensington.

Sarah Colborne, director of the Palestine Solidarity Campaign, said: "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 … 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."

The Labour MP Diane Abbott was among the speakers to address protesters as they gathered in Whitehall. "I am here to show solidarity with the people in Gaza," she said. "We call for an end to the occupation and justice for Palestinians."

Pro-Palestine supporters carried placards with the messages "Gaza, stop the massacre" and "stop Israel's war crimes".

Saturday's march follows on from a demonstration on Friday at the Cabinet Office by a group calling itself London Palestine Action. The protesters expressed their opposition to Israel's military action in Gaza by locking themselves to railings, unfurling banners and demanding to speak to the deputy prime minister, Nick Clegg, or a Cabinet Office official.

They posted images on their Facebook page of demonstrators linking arms as they sat in the lobby of the Whitehall building at around 4.30pm.

Police officers were called to remove the group about 25 people, who were chanting anti-Israel slogans and carrying banners reading "Stop arming Israel". Metropolitan police said no one was arrested.

Leila White, an art history student at King's College, took part in the protest with fellow activists, in response to the Israeli ground offensive that began in Gaza on Thursday. She said: "By allowing arms exports to Israel to flourish, the UK government is providing material and political support for Israel's violent oppression and collective punishment of Palestinians, including the current massacre in Gaza. "Basically, we want the UK government to invoke an arms Embargo against Israel.''

The group, which has previously staged flashmob demonstrations in the capital, describes itself as "a network of people in London taking creative action against Israeli apartheid through BDS [boycott, divestment and sanctions] and other effective, participatory Palestine solidarity work".

In a separate development, David Cameron and Barack Obama have spoken by phone to discuss the worsening situation in Gaza, according to Downing Street.

"Both leaders reiterated their support for Israel's right to take proportionate action to defend itself from the barrage of rocket attacks from Gaza", a No 10 spokeswoman said.

The two leaders also discussed Iran's nuclear programme and "agreed that we should keep up our efforts to negotiate a lasting agreement which gives the world confidence that the nuclear activities of Iran are exclusively for peaceful purposes".