Gordon Brown has added his voice to international calls for an immediate ceasefire in the Gaza Strip.
Mr Brown's appeal came after Israel rejected a European call for a two-day halt to air strikes which have killed 390 people and wounded 1,600 over the past five days.
Meanwhile, the Archbishop of Canterbury issued a plea for a new year "period of truce" in the Middle East, to allow the Israelis and Palestinians time to explore ways to restart the peace process.
In a statement released by Lambeth Palace, Dr Rowan Williams urged the outgoing US President George Bush and his successor Barack Obama to unite in a joint appeal for a cessation of hostilities.
Hundreds of demonstrators have gathered outside the Israeli Embassy in central London in a protest that will continue during Thursday, before moving on to the Egyptian Embassy on Friday and then to Trafalgar Square on Saturday.
Britain announced emergency aid worth £7 million to provide urgently-needed food and fuel to Gaza, where International Development Secretary Douglas Alexander said the humanitarian situation was "getting worse by the hour".
The PM said he had received assurances in a phone call with Israeli Prime Minister Ehud Olmert that there will be access for aid. United Nations aid workers are expected to go into the territory with supplies during Thursday.
"It is vital that moderation must now prevail - there's a humanitarian crisis," Mr Brown said.
He said he had talked to leaders on both sides about a set of proposals being discussed by the Arab League, which he said offered "the best way forward" for Gaza.
There have been demands for the House of Commons to return early from its Christmas break - not due to end until January 12 - to debate the Gaza crisis.
