Tens of thousands of people have taken part in a rally for Gaza in London.
The demonstration came amid renewed violence between Israel and Palestinian militants after a three-day ceasefire ended on Friday.
An emergency appeal in the UK for people affected by the conflict in Gaza has raised £4.5m in less than 24 hours, the Disasters Emergency Committee said.
Meanwhile, David Cameron and Barack Obama have expressed "serious concern" about the renewed hostilities.
The UK prime minister and US president, who discussed the ongoing fighting during a phone call, have both called for another ceasefire. They condemned Hamas for launching rockets from civilian areas and said Israel needed to show "restraint".
The Department for International Development (DfID) is sending a team of NHS medical experts to the region to help those injured in Gaza.
'Noisy but peaceful'
Saturday's demonstration, organised by the Stop the War coalition and the Palestine Solidarity Campaign, saw participants march past the US embassy on their way to a rally in Hyde Park.
Speakers there included Labour MP Diane Abbott, who told the crowd British people "stand in solidarity with the people of Gaza". She said it was the "biggest demonstration yet" about Gaza.
The BBC's Mark Sanders, at the rally, described it as "a very noisy protest but a peaceful one".
Protester Philip McCowen, 59, from Bristol, said: "The massacre of children is outrageous. The bombing of hospitals is outrageous. Collective punishment is a crime against humanity."
Yiftah Curiel, a spokesman for the Israeli embassy in London, told the BBC: "We don't have a problem with the protest per se; we have a problem with people expressing support for a terror organisation which is designated in the UK [Hamas] and which today is the key obstacle to the prosperity of Gaza."
More than 1,900 Palestinians, mostly civilians, have been killed in four weeks of Israeli attacks in the Gaza Strip, the UN says.
Sixty-seven people have died on the Israeli side, three of them civilians.
Israel says it is acting to protect its citizens from rockets fired from Gaza. Hamas, which controls Gaza, has demanded that Israel end its blockade of the territory.
'Day of rage'
The demonstration in London was part of a worldwide "day of rage" against Israeli military action in Gaza. In other demonstrations on Saturday:
- At least 50,000 people gathered in Cape Town, South Africa, organisers of the protest said
- Thousands marched through central Paris, in defiance of a ban imposed by French authorities
- About 2,000 people are estimated to have marched in Sydney and Melbourne, Australia
- Members of Jamiat Ulama-I-Hind organised a protest in the centre of Bangalore, India.
Other demonstrations were held in Spain, Greece, Jordan and Yemen on Friday.
The DEC launched its Gaza Crisis Appeal on Friday evening, with the British government saying it would match the first £2m of public donations.
Saleh Saeed, chief executive of DEC - an umbrella organisation bringing together 13 UK aid charities to deal with international crises - praised the "amazing generosity of the British public".
He said: "The funds are desperately needed, with ongoing fighting in Gaza creating an unbearable situation for families and children.
"Hundreds of thousands of people have been forced to leave their homes, the healthcare system is on the verge of collapse and many people have little or no clean water.
"Despite the end of the ceasefire, aid is getting through and many of our member agencies are still working on the ground. But with the scale of the humanitarian crisis in Gaza, they urgently need more money to scale up their life-saving work."
NHS staff including doctors, nurses and anaesthetists will fly out to the region in the next 48 hours.
Prime Minister David Cameron said: "The conflict in Gaza has taken a terrible toll. The UK has been at the forefront of humanitarian efforts to help those affected and it is right that we see what more we can do.
"The NHS has always stepped up to the plate to help those in need and this expert team will play a crucial role in helping hundreds caught up in this conflict."
'No military solution'
Conservative peer Baroness Warsi, who resigned from the government over its policy on Gaza, said arms export licences to Israel should be suspended.
She told BBC Radio 4's Today programme: "What we need to do right now is put all our efforts into making sure we move the government's position, that they suspend arms export licences immediately, that they start to lead the international effort on accountability on both sides and that they move towards a Middle East policy that is, in the long term, sustainable."
Hannah Weisfeld, director of Yachad, a UK-based Jewish group which supports a two-state solution, said many British Jews backed its call for peace.
She told the BBC: "Unless we can find a way to bring those two parties together - Israelis and Palestinians - and create a long-term political solution in which both peoples have the safety, security and self-determination that they deserve, then we're not going to get out of this current round of violence."
Human cost of the conflict
- 1,935 killed, including at least 1,408 civilians
- 452 children
- 235 women
- 64 soldiers
- 3 civilians, including one Thai national in Israel
(Source: OCHA; 08:00 GMT on 9 August)
The health professionals being flown out to help, who have volunteered their services, will initially be based with the charity Medical Aid for Palestinians at Al Mokassed hospital in East Jerusalem.
Tony Laurance, chief executive of the charity, who is leading a team of surgeons due to depart on various flights in the next 48 hours, told the BBC it was a "pretty awful situation", as hospitals in Gaza were "overwhelmed".
Although their main objective is to treat people in the region, Downing Street said some of the injured - particularly children - may need to be brought to the UK for treatment at specialist trauma centres.
Trauma surgeon Prof Chris Bulstrode, who is part of the team, said: "Our first job is to get on to the ground and talk to the people who are on the ground, listen to what they say their needs are, and then report back to make sure that we can put the right teams in to do the maximum amount of help."
He said the medics wanted to help local surgeons with equipment, training and how to deal with gunshot wounds and blast injuries.
The UK has already contributed £17m in emergency relief for Gaza.