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Scuffles broke out near Downing Street as protesters gathered in central London with banners and posters, some chanting "from the river to the sea, Palestine will be free", amid controversy over the slogan's meaning.
By Sunday, 11 people had been arrested, nine of whom were detained on Saturday, and five charged with public order offences.
Red and green flares were let off by protesters, with one woman knocked over as the sound of fireworks startled horses near the Palace of Westminster.
The Cenotaph war memorial was guarded by police as demonstrators filed past.
One person was arrested after an assault on a police officer, who was taken to hospital, although it was not clear whether the suspect was a protester or a counter-demonstrator.
Authorities were expecting about 100,000 people to take part as trade unionists and left-wing MPs such as former Labour Party leader Jeremy Corbyn joined calls for a ceasefire in Gaza.
Mr Corbyn called it an "eternal stain" that the UK abstained on a UN motion for a truce.
"It’s not much to ask, a ceasefire, when children are being killed by weapons coming through the rooms of their homes," he said.
UK Foreign Secretary James Cleverly said Hamas had given no sign it would abide by any ceasefire.
But Alicia Kearns, the chair of Parliament's foreign affairs committee, joined calls on Saturday for a temporary humanitarian truce.
"Hamas cannot be trusted, they are terrorists, but a truce is temporary, holding as long as hostilities are suspended for urgent humanitarian need," Ms Kearns said.
She said it was unlikely that the two sides would agree on terms on a full ceasefire.
Protest organisers in London, who say a similar event last week was the biggest ever pro-Palestine rally in Britain, urged activists to “make this one even bigger”.
The Metropolitan Police used public order powers to ban protesters from gathering in streets near the Israeli embassy.
Met commander Kyle Gordon said officers would “police up to the line of the law”, amid a debate over whether some pro-Palestinian slogans might amount to acts of incitement.
“Our most experienced and knowledgeable officers are working on the policing of these events, making sure we are utilising all legislation to its fullest extent,” Mr Gordon said.
London Mayor Sadiq Khan, one of the most prominent UK politicians to call for a ceasefire in Gaza, said the police force would do “everything in its power to tackle hate” at the rally.
Mr Cleverly said it was "perfectly possible to support the Palestinian people but also to condemn Hamas", but said there was a minority who had "much more negative aims" in the protests.
Downing Street said police had powers to crack down on “racism, intimidation and harassment” as it played down calls for tougher laws.
“We do believe at the moment police do have the powers to arrest those who incite violence or racial hatred,” a spokeswoman said this week.
The question of a ceasefire is causing divisions in UK politics, with Scottish First Minister Humza Yousaf leading calls for Britain to back a truce.
Mr Yousaf, who revealed he had lost contact with his family in Gaza and could “only pray they survive the night”, has written to party leaders urging them to call for a ceasefire.
Children laid teddy bears outside the Foreign Office in London on Friday in a protest to highlight the suffering of youngsters in Gaza.
In a separate action, 220 empty seats were set at a “Shabbat table” in north London in tribute to hostages being held captive by Hamas after its attack on Israel.
Prime Minister Rishi Sunak’s government abstained on a UN vote on a humanitarian truce in New York, saying it could not support a resolution that failed to condemn Hamas by name.
"As yet, I have seen or heard nothing from Hamas that gives me any confidence that they desire or would abide by calls for a ceasefire," Mr Cleverly told broadcasters.
The Labour front bench has also declined to support a ceasefire, saying the UK would have acted to defend itself if it had been attacked like Israel.
But prominent Labour figures including Mr Khan and Greater Manchester Mayor Andy Burnham have piled pressure on party leader Keir Starmer by coming out in favour of a ceasefire.
Mr Starmer, who has sought to repair Labour’s image among British Jews after Mr Corbyn’s tenure was dogged by allegations of anti-Semitism, has joined the government in backing temporary “humanitarian pauses”.