Members of the Sikh community gather in Trafalgar Square in central London on August 12, 2018 to call for a referendum of the Sikh global community to establish India's Punjab state as an independent country

Hundreds of Sikh separatists and their supporters gathered in London's Trafalgar Square on Sunday to demand a referendum on an independent homeland to be carved out of India.

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 protest was organised by the US and Canada-based group Sikhs for Justice, and drew people from all over the world, many of them chanting "Khalistan".

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."