Protesters wave flags during a pro-Palestinian protest in London, Saturday, Nov. 11, 2023. AP
The "National March for Palestine", organised by the Stop the War Coalition, set off after a two-minutes' silence to remember Britain's war dead on Armistice Day was observed at The Cenotaph war memorial in central London at 1100 GMT.
Huge crowds of protesters waved black, red, white and green Palestinian flags and held aloft placards proclaiming "Stop Bombing Gaza", as Israel's relentless bombardment of the strip has so far killed more than 11,000 people, mostly women and children.
As the march set off, protesters shouted "free Palestine" and "ceasefire now".
Saturday's march has generated concern because it coincides with Armistice Day, commemorating the end of fighting in World War I in 1918 -- and political criticisms about the policing operation.
Prime Minister Rishi Sunak made a late plea for calm late on Friday, calling for protesters to do so "respectfully and peacefully".
Met Police deputy assistant commissioner Laurence Taylor said march organisers had changed the route from Hyde Park to the US Embassy in south London to ensure it will not pass any landmark memorials.
Previous events have seen "small groups break away" and that "their behaviour has been escalating and becoming more violent", Taylor, who is leading the policing operation, told reporters.
Metal barriers have been placed around the area containing the most significant memorials, and an exclusion zone created, with police able to arrest any marchers who try to breach it.
Thousands of people wearing red poppies -- the symbol of remembrance -- stood heads bowed at The Cenotaph on Whitehall for a solemn ceremony of reflection, with other services held up and down the country.
About 1,850 police officers, including some from other forces across Britain, have been drafted in to keep the peace, with 1,375 on Sunday, when a national service of remembrance takes place at The Cenotaph led by King Charles III, senior royals and political leaders.
Home Secretary Suella Braverman, an increasingly outspoken right-winger, has done little to quell tensions, by accusing police of being more sympathetic to so-called left-wing protests than others.
About 80 counter-demonstrators have now been arrested at Tachbrook Street, BBC reported.
It appears there are a number of counter-protesters inside, sipping pints as the march outside continues south, it says.
The protest itself remains peaceful. It is being well marshalled and police officers - from various forces - appear to be containing situations well.
Aerial images over central London have captured the scale of the pro-Palestinian protest as it moves from Hyde Park to the US embassy in Vauxhall, south of the Thames.
Thousands of Jews are joining the huge march saying “Not in Our Name” and “Ceasefire Now.”