Rajya Sabha member Swapan Dasgupta was gheraoed twice and kept confined to a seminar hall in Visva-Bharati for over four hours by a group of students on Wednesday when he went there to deliver a lecture on the Citizenship (Amendment) Act.

The first gherao had been lifted around 9pm, after close to four hours, but soon the protesters returned and blocked Dasgupta and the others again for around 40 minutes.

As the gherao continued late into the night, the BJP issued a video warning to the Mamata Banerjee government.

Bengal BJP minder Kailash Vijayvargiya said in the video message that he was warning (chetavni) Mamata and her administration that if anything untoward happened to Dasgupta, “parinaam bohot kharab honge (the consequences will be very bad)”.

In the aftermath of a threat by some Left-wing students that they would not allow the programme, the Visva-Bharati authorities had changed the venue at the last minute, holding the lecture at its Sriniketan campus 3.5km from its main Santiniketan premises, but the protesters said they stopped the event midway.

“We would not let someone propagate the BJP’s Hindutva agenda at Rabindranath Tagore’s Visva-Bharati. We are happy that the programme could not be continued,” said a student, who did not wish to be identified, while taking part in the siege of the seminar hall of the social work department, where the event was shifted from the Lipika auditorium in Santiniketan.

Others who were not allowed to leave the hall included vice-chancellor Bidyut Chakrabarty and some Birbhum BJP leaders, besides a section of teachers and university employees.

The Visva-Bharati authorities had locked the main gate of the Sriniketan department compound where the lecture was to be held so that the protesters could not enter. But around 200 students broke the lock and reached the seminar hall building.

They locked the two doors of the seminar hall from outside and shouted slogans against the BJP. A section of students also locked the main gate of the department compound.

The protesters opened the windows of the hall as those attending the lecture tried to pull the curtains.

The protesters lifted the gherao around 9pm, but returned after around half an hour to resume the blockade. They finally allowed Dasgupta, a member of the university court, to leave past 10.30pm.

“Drama ends in Vishwa Bharati without the confrontation that some protesters desperately wanted,” Dasgupta tweeted.

The officiating public relations officer of Visva-Bharati, Anirban Sircar, said: “We could finish the programme and it was successful.”

VC Chakrabarty said Dasgupta was a “well-wisher of Visva-Bharati” and “what happened on Wednesday was not good for the university”.

Falguni Pan, unit secretary of the Revolutionary Students’ Front, a Left-leaning outfit, alleged that the lecture — titled “CAA 2019: Understanding and Interpretation” — was an attempt to promote “biased propaganda” at the central university.

Somnath Sow, an SFI leader at the varsity, said: “It is a shameful incident for Visva-Bharati when the authorities are organising a lecture by locking the gates…. If the authorities were neutral over the CAA, they could have organised a debate, not a one-sided lecture.”