Tens of thousands of protesters marched in support of Palestinians on Saturday in major European cities including London, Berlin, Madrid and Paris, as the worst violence in years raged between Israel and militants in Gaza.

In London, several thousand protesters carrying placards reading "Stop Bombing Gaza" and chanting "Free Palestine" converged on Marble Arch, near the British capital's Hyde Park, to march towards the Israeli embassy.

Packed crowds stretched all along Kensington High Street where the embassy is located.

Organisers claimed as many as 150,000 people had gathered for the London march, one of several across Britain, though London police said they were unable to confirm any figure.

Pro-Palestinian activists and supporters let off smoke flares, wave flags and carry placards in central London Pro-Palestinian activists and supporters let off smoke flares, wave flags and carry placards in central London Photo: AFP / Tolga Akmen

"The group is spread across a large area which makes it impossible to count them," a Metropolitan Police spokesman said.

"This time is different," Palestinian Ambassador Husam Zomlot told the demonstrators.

"This time we will not be denied any more. We are united. We have had enough of oppression."

Simon Makepace, a 61-year-old accountant told AFP he had joined the protests because "the whole world should be doing something about it, including this country".

A protester throws a metal bar during a demonstration in Paris A protester throws a metal bar during a demonstration in Paris Photo: AFP / GEOFFROY VAN DER HASSELT

He was critical of the United States, which he said was unfairly backing Israel, and urged Washington to "make peace and stop what's happening".

Azadeh Pyman, a 50-year-old scientist said she had been raised on the Palestinian cause by her parents and grandparents.

"I think it's the cause that will go from one generation to another generation, until Palestine is free," she said.

A woman sporting the word "Palestine" in Arabic marches in Madrid A woman sporting the word "Palestine" in Arabic marches in Madrid Photo: AFP / GABRIEL BOUYS

Later Saturday, two Leicester players, England's Hamza Choudhury and France's Wesley Fofana, held a Palestinian flag after their team won the FA Cup final.

Across North America, in turn, gatherings to show solidarity with Palestinians took place in cities including Boston, Washington, Montreal and Dearborn, Michigan.

Several hundred people turned out in the Bay Ridge area of Brooklyn, New York, chanting "Free, free Palestine" and "From the river to the sea, Palestine will be free."

They waved Palestinian flags and held placards that read "End Israeli Apartheid" and "Freedom for Gaza."

In Madrid, meanwhile, some 2,500 people, many of them young people wrapped in Palestinian flags, marched to the Puerta del Sol plaza in the city centre.

The Madrid march ended without incident The Madrid march ended without incident Photo: AFP / GABRIEL BOUYS

"This is not a war, it's genocide," they chanted.

"They are massacring us," said Amira Sheikh-Ali, a 37-year-old of Palestinian origin.

"We're in a situation when the Nakba is continuing in the middle of the 21st century," she said, referring to the "catastrophe", a word used by Palestinians to describe Israel's creation in 1948 when hundreds of thousands fled or were driven out.

"We want to ask Spain and the European authorities not to collaborate with Israel, because with their silence, they are collaborating," said Ikhlass Abousousiane, a 25-year-old nurse of Moroccan origin.

Police attempt to break up pro-Palestinian demonstration in Paris Police attempt to break up pro-Palestinian demonstration in Paris Photo: AFPTV / Emeline HENRY

The marches came amid the worst Israeli-Palestinian violence since a 2014 war in Gaza.

Thousands marched in Berlin and other German cities following a call by the Samidoun collective.

Three marches were authorised in Berlin's working-class Neukoelln southern district, home to large numbers of people with Turkish and Arab roots.

The protesters shouted "Boycott Israel" and threw paving stones and bottles at the police, leading to several arrests.

Other protests were held in Frankfurt, Leipzig and Hamburg.

On Tuesday, Israeli flags were burnt in front of two synagogues in Bonn and Muenster.

Police officers used tear gas and water cannon in Paris to try to disperse a pro-Palestinian rally held there despite a ban by authorities.

Some threw stones or tried to set up roadblocks with construction barriers, but for the most part police pursued groups across the district while preventing a planned march toward the Place de la Bastille.

The march had been banned on Thursday over concerns of a repeat of fierce clashes that erupted at a similar Paris march during the last war in 2014, when protesters took aim at synagogues and other Israeli and Jewish targets.

"France is the only democratic country to ban these demonstrations," said a statement from lawyers for the Association of Palestinians in the Paris region.

In Greece, police said around a thousand people marched on the US embassy in Athens. Riot police used water cannon and there were minor scuffles with protesters in front of the embassy, AFP correspondents reported.

In Rome, a few hundred people gathered near the Santa Maria Maggiore basilica, carrying large Palestinian flags and chanting slogans.

"No need to be Muslim to support the Palestinians," read one placard: "You just need to be a human being."

In Tunisia, demonstrations took place in several cities. Hundreds of demonstrators draped in Palestinian flags gathered in central Tunis, before marching on Habib Bourguiba Avenue, watched by police.

Israel is fighting Hamas militants in Gaza while trying to contain an outbreak of internal Jewish-Arab clashes and violence in the West Bank.

The Israeli bombardment began Monday, after Gaza's Islamist rulers Hamas fired rockets towards Jerusalem.

That was in response to bloody Israeli police action at the flashpoint Al-Aqsa mosque compound in Jerusalem, as well as a crackdown on protests against the planned Israeli expulsion of Palestinians from their homes in the Sheikh Jarrah neighbourhood in annexed east Jerusalem.

Palestinian armed groups have fired at least 2,300 rockets at Israel since, killing 10 people, including a child and a soldier. More than 560 Israelis have been wounded.