Among those expected to address the crowds are former Labour leader Jeremy Corbyn, Diane Abbott and Zarah Sultana.
It comes as 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.”