Tens of thousands of people took to the streets in Britain and in several other cities around the world on Saturday to call for an end to Israeli military action in Gaza.

In London a peaceful protest was organised by the Stop The War coalition. According to police, more than 20,000 people marched from the BBC's offices on Portland Place, via the US embassy to Hyde Park.

Families with children and members of Jewish groups were among the demonstrators, who carried Palestinian flags and banners calling for an end to the civilian deaths in Gaza. The crowed chanted "Free Palestine" and "Shame on you, Barack Obama", and a huge Palestinian flag was unfurled and held open for the benefit of the news helicopters above.

Rime Hadri, 34, a public health worker and volunteer for Stop the War coalition, said she decided to participate in the march because the government had "taken sides in the conflict".

"This has been ongoing for 60 years. I can see the injustice and inequality in this conflict," she said.

Lou Everett, a probation officer, said: "The US are sending arms to Israel; the UK's standing by, saying to Israel and Palestine 'you guys need to sort it out', whereas they need to be part of the solution."

There were also demonstrations in Manchester, Edinburgh and Dublin, where several hundred people demonstrated for the fifth Saturday in a row.

In South Africa an estimated 50,000 people took to the streets of Cape Town, led by Archbishop Desmond Tutu, in what the authorities called the biggest post-apartheid rally the city has seen. Demonstrators called for peace and appealed to President Jacob Zuma and the South African government to stop selling arms to Israel.

The demonstration in London comes in the wake of the chair of the parliamentary select committee on arms export controls, Sir John Stanley, writing to the foreign secretary, Philip Hammond, requesting details of any UK military exports that may have been used by Israel to attack Gaza.

On Friday night, Labour increased pressure on the government to suspend arms exports to Israel as the shadow foreign secretary, Douglas Alexander, called for the publication of a Whitehall review of existing licences.

Israel renewed strikes on Hamas on Saturday, with fighter jets carrying out 30 air raids, killing five Palestinians, after militants fired six rockets into the Jewish state.

The most recent ceasefire ended on Friday morning, but combat has not resumed with the same intensity, fuelling hopes for a new truce.

A US state department spokesperson said the Obama administration hoped that all parties would agree to another ceasefire.

Pressure group Peace Now has called on supporters to rally on Saturday night in Tel Aviv against the conflict and to call for a diplomatic solution.A public appeal has raised more than £4.5m in less than 24 hours to help the victims of Israel's attacks on Gaza.

The Disasters Emergency Committee (DEC) launched its Gaza crisis appeal on Friday night on all major UK television networks. The Department for International Development has matched the first £2m donated by the public pound-for-pound.

The funds will allow the committee's member agencies, such as Oxfam and the British Red Cross, to help the hundreds of thousands of Gazans needing clean water, food, shelter, medical treatment and psychological support.

Saleh Saeed, DEC chief executive, said: "The funds are desperately needed, with ongoing fighting in Gaza creating an unbearable situation for families and children. Despite the end of the ceasefire, aid is getting through ... but with the scale of the humanitarian crisis in Gaza, they urgently need more money."

Meanwhile, the Foreign Office minister who resigned this week in protest over the government's "morally indefensible" position on the Gaza conflict said Britain's approach had been flawed for some time.

Baroness Warsi told BBC Radio 4's Today programme that her objections went beyond David Cameron's refusal to condemn Israel over the civilian death toll.

Repeating her demand for an immediate suspension of arms exports to Israel, Warsi said there was no clear commitment from Britain to lead the international effort to keep both sides accountable.

"Our language was not there. It was lagging behind. I don't think it was just words that would have stopped me from doing what I did. I think it was a combination of issues. It was the language that we were using; I think it was our lack of support for international justice and accountability for the crimes that had been committed."

The Conservative peer said the government needed to "move towards a Middle East policy that is in the long term sustainable" and she cited the UK's decision to abstain when the UN general assembly voted in November 2012 to recognise a Palestinian state as symptomatic of the problems.

"There is no point in us talking about a two-state solution if we don't do the simple things like recognising Palestine in the way that the majority of the world has at the UN," she added.

Warsi dismissed complaints that she had failed to condemn thousands of rocket attacks on Israel by Hamas, or the actions of Islamic extremists in Iraq.

Her resignation meant she could "live with myself", and she hoped it would bring together a "broad coalition" to address the issues behind the conflict.