The UK government will take the security of the Indian High Commission here "seriously", top British officials have said as they condemned as "disgraceful" and "completely unacceptable" the vandalism at the mission by a group of protesters waving separatist Khalistani flags.

The tricolour flying atop the Indian High Commission was grabbed at by the protesters chanting pro-Khalistani slogans on Sunday, leading to an arrest related to the violent disorder.

Officials from the mission said the "attempted but failed" attack had been foiled and that the tricolour was now flying "grander".

The Metropolitan Police said two members of security staff sustained minor injuries which did not require hospital treatment. An investigation has been launched.

Responding to the incident, London's mayor Sadiq Khan said he condemned "the violent disorder and vandalism that took place".

"There is no place in our city for this kind of behaviour," he tweeted.

The British High Commissioner to India Alex Ellis described the incident as "disgraceful" and "totally unacceptable".

Foreign Office minister Lord Tariq Ahmad of Wimbledon said he was "appalled" and the government would take the security of the Indian High Commission "seriously".

"This is a completely unacceptable action against the integrity of the Mission and its staff," he tweeted.

Scotland Yard said it was called to reports of disorder on Sunday afternoon and that a man has been arrested as its enquiries continue.

"Windows were broken at the High Commission building,” the Metropolitan Police statement said.

“Officers attended the location. The majority of those present had dispersed prior to the arrival of police. An investigation was launched, and one male was arrested nearby a short time later on suspicion of violent disorder. Enquiries continue,” the statement said.

India, meanwhile, registered its strong protest with the British government over the safety of its diplomatic mission and questioned the lack of sufficient security at the premises.

Images of shattered windows and men climbing the India House building were circulating on social media and videos from the scene show an Indian official grabbing the flag from a protester through the first-floor window of the mission, while the protester is seen hanging off its ledge and waving a Khalistan flag before it is taken away.

The Ministry of External Affairs (MEA) said the senior-most UK diplomat in New Delhi was summoned late evening on Sunday to convey India’s “strong protest” at the actions taken by separatist and extremist elements against the Indian High Commission in London.

“An explanation was demanded for the complete absence of British security that allowed these elements to enter the High Commission premises. She was reminded in this regard of the basic obligations of the UK Government under the Vienna Convention,” the MEA statement said.

India finds unacceptable the indifference of the UK government to the security of Indian diplomatic premises and personnel in the UK, the statement said.

"It is expected that the UK Government would take immediate steps to identify, arrest and prosecute each one of those involved in today’s incident, and put in place stringent measures to prevent the recurrence of such incidents,” it said.

The banned terrorist organisation, Sikhs For Justice, is conducting a so-called "Referendum 2020" amid a crackdown on pro-Khalistan leader Amritpal Singh in Punjab.

Sunday's incident came after similar radical actions by Kahlistani supporters in Canada and Australia.

Last week, the Honorary Consulate of India in Australia's Brisbane city was forced to close down on Wednesday due to safety concerns after Khalistan supporters organised an unauthorised gathering and blocked the entry of the office, days after Australian Prime Minister Anthony Albanese assured India that his government will not tolerate "extreme actions." Several Hindu temples in Melbourne were vandalised by Khalistan supporters in recent months.

India has repeatedly raised the issue with Australian authorities.

India has asked the Australian government to curb the anti-India activities of the Khalistani separatists and also attacks on the Hindu temples in the country.

Australian Prime Minister Albanese during his India visit assured his Indian counterpart Narendra Modi that Australia won’t tolerate any extreme actions like attacks on religious places.

Canada has also seen a rise in anti-India activities recently by Khalistani supporters who have vandalised some Hindu temples.

Last September, the Ministry of External Affairs issued a statement condemning the rise of hate crimes against Indians and anti-India activities in Canada, expressing their concern with stern language.