They brandished banners reading "Free Punjab, End Indian occupation", "Punjab Referendum 2020 for Khalistan" and "We will re-establish Punjab as an independent country".

The Indian government has expressed concern to London over the activity of expatriate Sikh separatists, who want to create a homeland of Khalistan in India's northern Punjab state.

The Foreign Office confirmed it discussed the rally with the Indian High Commission, but a spokeswoman told AFP: "Peaceful protest is a vital part of a democratic society."

Gurpatwant S. Pannun, legal advisor to Sikhs for Justice, said: "This is a peaceful, democratic campaign to give Sikhs the right to determine their own future."

Jas Singh, 26, a demonstrator from London, told AFP: "In India, anybody who speaks for a referendum gets picked up and put in jail."

There was a small counter-demonstration of about a dozen people singing and holding up signs saying: "We stand for one united India."