NEW DELHI - Indian police have opened an investigation into a protest this week at its High Commission in London, Reuters partner ANI reported on Friday, pursuing action on an incident that has sparked tension in relations with Britain.

Protesters with "Khalistan" banners took an Indian flag down from a first-floor balcony of the High Commission in the British capital on Sunday to denounce recent police action in India's Punjab state, British and Indian media reported.

Khalistan is the name of an independent Sikh homeland that some members of that community aspire to, both at home in India and in countries where Sikhs have settled.

The Indian foreign ministry said that it had shared the incident report with relevant agencies and now it was up to law enforcement agencies to pursue it.

"Whenever we feel there is an issue related to the security of our missions abroad, high commissions or embassies, this issue is taken up," External Affairs Ministry Spokesperson Arindam Bagchi said.

"We expect the host governments will take action to identify and prosecute all those involved. And take necessary measures to prevent the recurrence of such incidents."

India was not interested in assurances but would like to see action, he added.

India summoned the top British diplomat in New Delhi on Sunday to convey its "strong protest at the actions taken by separatist and extremist elements" at the mission, and to seek an explanation for "the complete absence" of security there.

British Foreign Secretary James Cleverly said acts of violence towards staff at the High Commission in London were unacceptable and British police were investigating.

Indian police had also opened an investigation, the ANI news agency reported.

Police officials and the British embassy in New Delhi did not immediately respond to calls and messages seeking comment.

Delhi police also removed some barricades protecting the British High Commission and the British High Commissioner's residence on Wednesday. Asked about it, Bagchi said he could not comment on the security of foreign missions in India.

Sikh militants complaining of unfair treatment on the part of the central government and seeking an independent homeland waged an anti-government campaign in the 1980s and early 1990s in which tens of thousands of people were killed.

Police in Punjab, where Sikhs are in the majority, this month launched a hunt for a Sikh preacher who has revived talk of an independent homeland.

(Reporting by Sakshi Dayal; Additional reporting by Krishn Kaushik; Editing by Robert Birsel, YP Rajesh and Nick Macfie)