Authorities in India extended a mobile internet blackout across the northern state of Punjab on Monday as police hunted for radical Sikh preacher Amritpal Singh.

The 30-year-old has risen to prominence in recent months for his push to create a separate Sikh state known as "Khalistan."

Authorities say the internet blackout will remain in place until at least noon on Tuesday.

Police in Punjab arrested more than 100 of his supporters following protests in the Sikh-majority state over the weekend. But Singh's whereabouts remain unknown.

What is the Khalistan movement?

The push for a breakaway Sikh homeland enjoys some support in Punjab, where about 58% of the population of 30 million are Sikhs.

The Khalistan movement is banned in India, where the government considers it to be a national security threat.

Thousands of people died during earlier efforts to advance the Khalistan separatist campaign in the 1980s and early 1990s.

It is also a popular idea among some members of the Sikh diaspora in countries such as Canada, the UK and the US.

India has appealed to foreign governments to take action against Sikh radicals in the Indian diaspora who they accuse of trying to revive the insurgency.

Attacks in London, San Francisco

Police in the UK reported on Monday that Khalistan supporters had allegedly smashed windows at the Indian High Commission in London over the weekend and removed the building's Indian flag. 

The Indian government said it had summoned British deputy high commissioner in New Delhi to demand an explanation.

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

UK Foreign Office Minister Tariq Ahmad said he was "appalled" by the incident in London.

"This is a completely unacceptable action against the integrity of the Mission and its staff," he wrote on Twitter. "The UK government will always take the security of the Indian High Commission seriously."

Similar unrest was reported at the Indian Consulate in San Francisco. Footage posted online appeared to show Singh's supporters smashing the consulate's doors and windows after breaking down barricades. An official cited by the AFP news agency said the Indian Foreign Ministry was closely examining the video.

nm/rt (AFP, AP)