New Delhi has accused London of failing to fulfill “basic obligations” to protect foreign diplomatic missions under the Vienna Convention, after a protester scaled the balcony of the Indian embassy and tried to replace the national flag with a separatist banner.

India’s foreign ministry summoned deputy chief of the UK high commission, Christina Scott, on Sunday evening to lodge a protest and demand explanations “for the complete absence of the 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 foreign ministry said in a statement, calling the alleged “indifference” of British authorities to the security of Indian diplomatic personnel “unacceptable.”

A group of protesters rallied outside of India’s High Commission in London on Sunday, according to videos that emerged on social media. The separatists, who belong to the Sikh faith, have been fighting for the right to establish a sovereign Khalistan in the north Indian state of Punjab, bordering India’s arch-rival and nuclear-armed neighbor Pakistan.

At some point one of the protesters scaled the second-floor balcony of the building and pulled down the Indian flag – before the embassy security guard swooped in to reclaim the national symbol. The activist then scuffled with the staffer for a while, in an apparent attempt to raise the Khalistan banner instead.

By the time Metropolitan Police officers arrived at the scene, the “majority of those present had dispersed,” but one man was arrested “on suspicion of violent disorder,” authorities said. 

“Windows were broken at the High Commission building… Two members of security staff sustained injuries. These are believed to be minor; they did not require hospital treatment,” a police spokesperson told local media, adding that authorities launched a further investigation into the incident.

British high commissioner, Alex Ellis, who wasn’t in New Delhi to take the heat off his deputy, condemned the “disgraceful acts” in a tweet, calling the events in London “totally unacceptable.”

New Delhi went on to say it “expected that the UK Government would take immediate steps to identify, arrest and prosecute each one of those involved in today's incident.”