A group of Britain's most prominent Jews has called on Israel to cease its military operations in Gaza immediately, warning that its actions, far from improving the country's security, will "strengthen extremism, destabilise the region, and exacerbate tensions inside Israel".

Describing themselves, as "profound and passionate supporters" of Israel - and supporting its right to defend itself against the "war crime" of Hamas rocket attacks - they added that the current tactics threatened to undermine international support for Israel.

The intervention, in a letter published in today's Observer, came as fears grew that Israel was to launch a "new phase" of its military offensive inside the Gaza strip. Yesterday warplanes dropped leaflets warning Gazans "not to be close to terrorists, weapons warehouses and the places where the terrorists operate". The two-week-old campaign has already killed more than 800 Palestinians, while 13 Israelis have died, three of them civilians killed by Hamas rockets.

Although individual Jewish writers and religious figures have expressed their opposition to the conduct of Operation Cast Lead, the letter represents the most significant break with Israel's tactics from a group of UK Jews.

Prominent rabbis, academics and political figures are among the signatories, including Rabbi Dr Tony Bayfield, head of the Movement for Reform Judaism; Sir Jeremy Beecham, former chair of the Labour party; Professor Shalom Lappin of the University of London; Baroness Julia Neuberger; Rabbi Danny Rich, chief executive of Liberal Judaism; Rabbi Professor Marc Saperstein, principal of Leo Baeck rabbinical training college; and lawyer Michael Mitzman, who set up Holocaust Memorial Day Trust for the Home Office.

Their demand comes amid increasing pressure on Israel from the diplomatic community to halt its operations, and rising criticism of the humanitarian impact on Palestinian civilians, including allegations of potentially serious breaches of international humanitarian law. Demonstrations around the world yesterday called for a ceasefire.

"We look upon the increasing loss of life on both sides of the Gaza conflict with horror," reads the letter. "We have no doubt that rocket attacks into southern Israel, by Hamas and other militant Palestinian groups, are war crimes against Israel. No sovereign state should, or would, tolerate continued attacks and the deliberate targeting of civilians. Israel had a right to respond and we support the Israeli government's decision to make stopping the rocket attacks an urgent priority.

"However, we believe that now only negotiations can secure long-term security for Israel and the region."

The letter was written before the escalation of ground fighting in Gaza City itself signalled by Israel yesterday.

"There can be no alternative to a negotiated solution," said Beecham. "Israel should be demonstrating, along with the Palestinian Authority, that there are economic and political benefits to be gained from peaceful engagement rather than violent confrontation."

His sentiments were echoed by Lappin: "Relying on overwhelming military force to respond to terrorist provocations invariably imposes horrendous suffering on innocent Palestinian civilians while entrenching the agents of terror in their midst. We have no alternative but to pursue rational, long term political options that promote moderation and marginalise extremists."

In London violent clashes broke out near the Israeli embassy as tens of thousands marched in protest. Helmeted riot police with batons and shields charged a group of demonstrators who hurled sticks, shoes and traffic cones back at them while chanting "Free Palestine!"

Protesters tried to force entry to the north gate of Kensington Palace Gardens and six climbed an adjoining wall, setting fire to an American flag. The windows of a Starbucks opposite the embassy were smashed.

The police charges created waves of panic. Protester Ahmed Mohammad, 23, claimed he saw women and children get hurt: "It was a peaceful protest until the riot police came. I've seen a mother and little girl pushed to the ground."

Some protesters attempted to throw barriers and other missiles at police.

The Stop the War Coalition, which organised the event, claimed that "at least" 100,000 people had made it "the biggest demonstration of solidarity with the Palestinian people in the history of this country". The Metropolitan Police estimated the total at 12,000.

Earlier, Speakers' Corner at Hyde Park was turned into a sea of Palestinian flags and banners condemning Israel. Speakers included human rights advocate Bianca Jagger, singer Annie Lennox and the Rev Garth Hewitt, canon at St George's Cathedral in Jerusalem.