Unidentified men hurled "inflammable substance" near the site of anti-citizenship law protest in Shaheen Bagh but none of the five women protesters present there were injured, police said.
However, the protestors alleged that the attackers came on motorcycles and hurled petrol bombs.
The incident happened around 9.15 am when entire Delhi was observing a 'Janta curfew' with people staying indoors in a bid to contain the spread of the novel coronavirus.
Police found a banner, rugs, burnt bamboo and a few broken bottles. Some bottles were also found in a lane near the protest site, said Deputy Commissioner of Police (Southeast) RP Meena.
Initial investigation revealed that a person set a banner on fire after "putting inflammable substance using a lighter," Meena said, adding a case has been registered at Shaheen Bagh police station.
One of the volunteers who did not wish to be identified said the men threw a petrol bomb in a lane near the protest site. "They had petrol bombs but there were not many people around then. Only five women were sitting on protest," he said.
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He said there has been a difference of opinion between two groups over continuing the protest and police had to be called in when they fought on Saturday. A police official said they suspect an insider's involvement in Sunday's incident as the protesters had clashed on Thursday too.
"We were informed about a fight at the protest venue and had to intervene to sort out the matter," he said.
A section of protestors want to end the agitation due the coronavirus outbreak but others want to continue. Only five women now sit on protest and many have left their slippers there as a symbol of their solidarity.
"We are following the guidelines related to the coronavirus. We have reduced our gathering to the permissible number. We have been cleaning and sanitising the venue," another volunteer said.
A woman protester said a fire had broken out near the barricades and police were called after the incident on Sunday.
"It is unfortunate that such attacks are being planned to disrupt our peaceful protest. We have followed all the guidelines related to coronavirus. Only five women are sitting at the site and maintaining a distance. We have been washing our hands and maintaining utmost hygiene. Such incidents are being done to move us from our protest," she said.