It comes amid reports that an Israeli air strike destroyed a high-rise building that housed the Associated Press (AP), Al-Jazeera and other media groups in the Gaza Strip.
The AP said the air strike came roughly an hour after the Israeli military ordered people to evacuate the building.
The news agency said there was no immediate explanation as to why the building was targeted.
Thousands of Palestinians have been forced to flee from their homes after a week of sustained conflict.
Since Monday night, Palestinian militant group Hamas has fired hundreds of rockets into Israel, whose military responded by barraging the Gaza Strip with tank fire and air strikes.
At least 126 people have been killed in Gaza, including 31 children and 20 women.
In Israel seven people have been killed, including a six-year-old boy and a soldier.
The demonstration in London was organised by Palestine Solidarity Campaign, Friends of Al-Aqsa, Palestinian Forum in Britain, Stop The War Coalition, Campaign for Nuclear Disarmament and the Muslim Association of Britain.
A spokesperson for the organisers said: “It is vital that the UK Government takes immediate action.
“It must stop allowing Israel’s brutal violence against and oppression of the Palestinian people to go unpunished.
“The bombardment of Gaza which is killing civilians including children is a war crime.
“The UK Government is complicit in these acts as long as it continues to offer Israel military, diplomatic and financial support.”