Part of Scotland Yard's public website was disrupted as thousands of masked protesters descended on central London for the Million Mask March organised by hacking group Anonymous.
Wearing characteristic Guy Fawkes masks, supporters of the mysterious online collective crowded into Trafalgar Square with scores of police on hand to keep order.
The Metropolitan Police said "the majority of those taking part were peaceful" but there were "pockets" of disorder and multiple fireworks thrown, though they did not inflict injury.
By its conclusion, police had made 53 arrests, the majority for drug offences and obstruction of officers.
The Met are investigating what caused the news section of its website to crash between Saturday evening and Sunday morning.
A spokesman said: "During the time the Met News web address was not working - between the evening of Saturday, 5 November and the morning of Sunday, 6 November.
"We have contacted our supplier regarding the temporary disruption to the usual Met News web address.
"There are no security issues with the Metropolitan Police Service website."
The main site was unaffected and the news site was now working, he added.
Saturday's protest began peacefully, with several participants climbing the base of Nelson's column and chanting the slogan "one solution, revolution".
The procession began along Whitehall and there were angry scenes as police formed a ring of steel outside Parliament.
There were chaotic scenes as riot police moved in to make an arrest in Trafalgar Square near the 9pm curfew, and several bottles and fireworks were thrown at police lines.
Scotland Yard imposed restrictions on this year's event amid concerns over disorder, limiting the march to a three-hour period between 6pm and 9pm on a prescribed route between Trafalgar Square and Whitehall.
Four officers and six police horses were injured after missiles and fireworks were thrown at them during last year's march.
Commander BJ Harrington, of the Met's Public Order Command, said: "I would like to commend my officers policing this challenging event for their professionalism and faultless co-ordination.
"I would also like to thank those participants who conducted themselves in a peaceful manner as well as Londoners and those visiting our city for their patience and tolerance."