Tens of thousands of protesters have marched through the streets of cities around the world demanding an end to Israel's offensive against Gaza, and a firmer position from the international community against the violence.

From London to Paris, and from Tehran to Cape Town, demonstrators gathered on Saturday to denounce the Israeli operation, while calling for an end to the occupation of the Palestinian territories in Gaza and the West Bank.

The protests come as former Cuban president Fidel Castro signed an international manifesto in Havana "supporting Palestine" and demanding that Israel respect UN resolutions and end the occupation.

The pursuit of the people of Palestine for freedom is something that is unstoppable, because it is a righteous cause. - Desmond Tutu, Anglican bishop and anti-apartheid campaigner

In London, thousands marched through the city shouting anti-Israel slogans and waving banners calling for an "end to the siege".

Al Jazeera's Nadim Baba, reporting from the UK capital, said that demonstrators first marched to the headquarters of the BBC, to denounce what they alleged as Israeli bias in its news coverage.

Demonstrators then marched to London's Hyde Park, where they also called on the government to impose an embargo on the sale of weapons to Israel.

David Cameron, Britain's prime minister, is coming under increasing pressure over his government's policies on Gaza, and the internal refugee crisis there.

In response, Cameron announced that his government will send medical volunteers to help the injured in Gaza, and contribute $20m in emergency relief.

'Immediate ceasefire'

In Paris, demonstrators unfurled banners condemning the violence, in which an estimated 1,911 Palestinians have died.

Some demonstrators carried mannequins wrapped in Palestinian flags, each representing a dead Palestinian.

The conflict has prompted high emotion in France, home to Western Europe's largest Muslim population and largest Jewish community.

In the German capital Berlin, more than 1,000 people took part in at least two rallies, which ended with a demonstration at a central square.

In a joint statement issued on Friday, French Foreign Minister Laurent Fabius, his German counterpart Frank-Walter Steinmeier and British Foreign Secretary Philippe Hammond called for an "immediate ceasefire" in Gaza.

Tutu condemnation

In South Africa, Anglican Archbishop Desmond Tutu, one of South Africa's most prominent anti-apartheid leaders, joined thousands of people in Cape Town to denounce the continued bombardment.

"The pursuit of the people of Palestine for freedom, for freedom from humiliation, from persecution, from the unjust policies of Israel - that is something that is unstoppable, because it is a righteous cause," he said.

In Havana, Castro, the country's former leader who will be 88 next week, added his signature to those of intellectuals and politicians to the document "In Defence of Palestine", the official Granma newspaper said on Saturday.

Bolivian President Evo Morales, Argentine artist and Nobel Peace Prize winner Adolfo Perez Esquivel, Cuban dancer Alicia Alonso and US writer Alice Walker were also among the signatories.

In Tehran, hundreds of doctors, paramedics and nurses gathered in the Iranian capital's "Palestine Square" to express their willingness to go to Gaza and treat the Palestinians injured in Israel's attacks.

Holding banners depicting dead Gazan children and Palestinian flags, the demonstrators shouted: "Death to Israel."

Israeli air strikes struck the Gaza Strip on Saturday, killing a senior Hamas member, as rocket fire continued following the collapse of a three-day truce aimed at ending the war between Israel and Hamas.