Nearly 2,000 police were out in force to keep rival groups apart, with the march organized on Armistice Day, the annual event when Britain remembers its war dead with solemn ceremonies at war memorials.
The march went ahead after a week of tensions, which saw the government call for the it to be scrapped, and police said they made scores of arrests.
Some 150 people from the mass protest were detained under public order legislation for wearing face coverings and setting off fireworks, while 82 counter-protesters were held to prevent them infiltrating the main march.
Groups of men, many wearing black with their faces covered and waving England's St George's flag and the Union Jack, tried to break through police lines at The Cenotaph war memorial on Whitehall.
Police in riot gear then faced a barrage of bottles in nearby Chinatown, the Metropolitan Police said.
"I condemn the violent, wholly unacceptable scenes we have seen today from the EDL (English Defense League) and associated groups and Hamas sympathizers attending the 'National March for Palestine,'" said Sunak in a statement.
"The despicable actions of a minority of people undermine those who have chosen to express their views peacefully."
Sunak, who has resisted calls for him to back a ceasefire in Israel's war with Hamas, said far-right "thugs," anti-Semitic chants and pro-Hamas signs and clothing had marred remembrance weekend.
"All criminality must be met with the full force of the law," he added.
Scuffles
The march, organized by the Stop the War Coalition, is the biggest yet in London since Hamas's attack killed more than 1,200 and took some 240 people hostage on October 7, according to Israel.
The Israeli military campaign in response has left just over 11,000 people in Gaza dead, according to the Hamas-run health ministry in the coastal enclave.
Huge crowds waved black, red, white and green Palestinian flags and held aloft placards proclaiming "Stop Bombing Gaza," shouting "free Palestine," "ceasefire now" and "Israel is a terror state."
Police estimated that 300,000 people had turned out, while organizers put the figure at 800,000, putting it on a par with the huge numbers who marched in the British capital against the Iraq war in 2003.
"Forget the political stance, forget everything else, you can't stand around while people are getting killed," Shiraz Bobra, 41, who had traveled from Leicester, central England, told AFP, adding he would come every week until a ceasefire was enforced.
Roman Catholic priest Father John McGowan added: "I feel for the Palestinians because their land is occupied and their occupiers can be cruel" and said he hoped for a two-state solution.
The number of arrests on Saturday topped those from all previous pro-Palestinian marches combined, which have seen people detained for hate crimes and showing support for Hamas, which is proscribed as a terrorist group in the UK.
Police said they were prepared for small breakaway groups and expected pockets of violence, with concern about far-right groups, including football hooligans massing to protect landmark memorials.
The founder and former leader of the far-right EDL, Tommy Robinson, was seen among the crowds of counter-protesters.
The Metropolitan Police said it was "actively seeking" two masked men pictured on the march wearing Hamas headbands, promising "proactive action when they are identified".
Anti-Semitic slogans were spotted among the placards, British media reported.
'Hate marches'
About 1,850 police officers, including some from other forces across Britain, have been drafted in to keep the peace.
An exclusion zone was created around central London, including The Cenotaph, where crowds wearing poppies -- the symbol of remembrance -- gathered to pay their respects in silence, and by laying wreaths.
King Charles III leads a national remembrance event at the war memorial with senior royals, political leaders and veterans on Sunday.
Home Secretary Suella Braverman, an increasingly outspoken right-winger, did little to quell tensions this week, by accusing police of being more sympathetic to so-called left-wing protests than others, and calling the pro-Palestinian demos "hate marches."
Support for Palestinians is a long-standing policy of the British political left.
Her political opponents accused her of loose talk that emboldened far-right protesters to try to take on the police, and enraging pro-Palestinian supporters.
There were other pro-Palestinian rallies elsewhere in Europe. with several thousands turning out in Paris and more than 20,000 in Brussels.
Some of the marchers in the Belgian capital cried out "EU, shame on you" for perceived bias towards Israel at the expense of Palestinian lives and rights.
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