More than 100 people were arrested after street battles on Tuesday that forced Amit Shah, president of the ruling Bharatiya Janata Party (BJP), to leave an election rally in the eastern capital of West Bengal state under armed guard, police said.

The Election Commission on Wednesday ordered an end to campaigning at 10 pm on Thursday, a day early, in West Bengal for Sunday's final election round.

Separately a grenade in the city of Guwahati in the north-eastern state of Assam injured six people, police said. A local separatist group, the United Liberation Front of Asom (Ulfa), claimed responsibility.

In Tuesday's disturbances in Kolkata, a bust of a renowned Bengal reformer, Ishwar Chandra Vidyasagar, was destroyed and a college named after him ransacked, which the BJP and rival Trinamool Congress (TMC) angrily blamed on each other.

Teachers and students of Vidyasagar college wearing black badges staged a day-long protest Wednesday, demanding an impartial investigation.

"I could not recognize the office when I stepped in today," Gautam Kundu, the college principal, told AFP. "Broken shards of glass lay all over and a statue of Vidyasagar was missing from the glass case.

"They (the teachers) were cowering in fear inside the classrooms when the attack took place. We are shocked," he said.

Security was stepped up outside the college as well as at Calcutta University, where students demonstrated with placards reading "Go Back Narendra Modi", "Go Back BJP".

Modi's BJP has campaigned hard in West Bengal, hoping to capture many of the state's 42 parliament seats from the TMC to compensate for losses predicted in other regions.

The BJP accused West Bengal's chief minister Mamata Banerjee of orchestrating attacks on Shah's roadshow. Government ministers joined a protest in Delhi which blamed Banerjee's party.

West Bengal state has been hit by repeated violence between the two parties during the six-week-long election that ends Sunday with results due on May 23.

As in previous elections, there has been also sporadic violence elsewhere.

On May 1 Maoist rebels fighting a decades-old insurgency against the Indian state killed 15 Indian troops and their driver in the in the western state of Maharashtra.

In Wednesday's explosion in Guwahati, two men came on a motorcycle and threw a grenade in front of the shopping mall, police said.

Activist freed

"Has Bengal acquired a government of gangsters? The attack on Amit Shah's peaceful rally by TMC is deplorable," Finance Minister Arun Jaitley said on Twitter.

Modi condemned what he called "TMC goons" for launching the street battles, in a television interview.

Banerjee hit back by accusing Shah acting like a "god" who does not allow protests.

She led a rally of several thousand supporters through the city late Wednesday. The communist party held its own rally.

Trouble erupted when Shah's convoy drove past the entrance to Calcutta University, where TMC student wing activists had gathered.

"TMC supporters started throwing stones at the tail-end of the rally. A group of people then set two motorbikes and a bicycle on fire," said witness Partha Sinha.

Political observers say the aggressive campaign by Modi's party against the TMC in the eastern state has made it one of the most bitter contests in the election.

Modi won a landslide victory in 2014, but many pollsters expect him to lose his overall majority in this election. A good performance in West Bengal would significantly improve his chances of a second term.

The BJP and Banerjee's party have also been at loggerheads over a BJP activist arrested for posting a doctored image on Facebook with Banerjee's head on the body of Bollywood star Priyanka Chopra wearing a revealing dress.

Priyanka Sharma was arrested last week for posting the viral meme that sparked national headlines.

She was released on Wednesday after the Supreme Court intervened.

But Sharma told journalists after her release: "All I did was post a meme and I was jailed for 5 days. I am not sorry."