Almost 2,000 Khalistani protesters were seen at the spot, the London police said.
Khalistan supporters protesting against the crackdown on Khalistani leader Amritpal Singh on Wednesday, shouted slogans, threw water bottles and ink at the London Metropolitan police, who kept them confined to the other side of the road, a safe distance from the Indian High Commission. For the protesters, the immediate provocation was the bigger Indian flag draped over the walls of India House. The High Commission employees had retaliated with a bigger flag a day after Sunday's unprecedented vandalism, when the Indian flag was pulled down and the windows of the building were broken.
The London police had appeared ready for the protesters' show of strength today, deploying forces in 24 buses as well as the mounted police.
While the protest started small, the numbers grew as the evening advanced. By late evening, around 2,000 protesters had turned up at the spot, the police said. The mood got ugly as they tried to break the barricade and targeted the police with water bottles, ink and powdered colours. The police said they will evacuate the spot if the protest escalates any further.
The extra security in London came shortly after the police in New Delhi removed traffic barricades outside the British High Commission, in a move interpreted by some as a demonstration of India's displeasure with the breach in London. The police have explained the move as removal of barricades that were "creating hurdles" for commuters.
Late on Sunday evening, India summoned a senior British diplomat in Delhi to register its strong protest over the "complete absence of British security" as the crowd targeted the building, protesting against the crackdown on Khalistani leader Amritpal Singh and his group.
"An explanation was demanded for the complete absence of the British security that allowed these elements to enter the High Commission premises," the foreign ministry had said.
The heightened security came ahead of a planned protest on Wednesday.
The ministry also said that the UK Government is expected to "take immediate steps to identify, arrest and prosecute each one of those involved in the incident," and put in place stringent measures to prevent a recurrence.
While British officials condemned the vandalism, calling it "disgraceful" and "completely unacceptable", only one person was arrested by Scotland Yard over the incident. The person is now out on bail.
On Wednesday, another protest was held by pro-Khalistan groups who circulated invitations on WhatsApp. The Federation of Sikh Groups, which had signed some of the invitations, was unavailable for comment.