West Bengal Chief Minister Mamata Banerjee yesterday urged all Indian opposition parties to unite against ruling BJP, a day before she clashes in Nandigram in a vote battle against one of her former confidante who defected to BJP, which has thrown a serious challenge to her 10 year rule in the crucial Indian state. 

Indian authorities yesterday banned large gatherings in the violence-hit constituency which has seen rival parties clash in the streets resulting in deaths in build up to the election.

Today's second day of polling in West Bengal state, home to 90 million people, is central to a massive push by Prime Minister Narendra Modi's Hindu nationalist party to oust Mamata, a fierce critic of the government.

Mamata, a 66-year-old firebrand whose Trinamool Congress party ended three decades of communist rule in the state in 2011, is contesting the election in Nandigram against Suvendu Adhikari, who defected to BJP last year.

The high-stakes showdown in a state notorious for political violence saw the Election Commission ban gatherings of more than four people in a bid to ensure safety for voting, officials said.

"It's battle royale in Nandigram," an election official in Kolkata told AFP.

Just a day ahead of the voting day, Mamata yesterday wrote to ten key opposition leaders including Congress's Sonia Gandhi, suggesting a big get-together for a strategy to take on the BJP after the current round of assembly polls, reported NDTV.

The strongly worded seven-point letter said the time has come for a "united and effective struggle against the BJP's attacks on democracy and the constitution" and "presenting a credible alternative to the people of India".

Starting with the controversial new law that gives more powers to Delhi's Lieutenant Governor, the Centre's representative, compared to the city's elected government,  Mamata presented seven instances of what she called the BJP's "assaults" on democracy and cooperative federalism.

"The BJP wants to make it impossible for non-BJP parties to exercise their constitutional rights and freedoms. It wants to dilute the powers of the state governments and downgrade them to mere municipalities. In short, it wants to establish a one-party authoritarian rule in India," she wrote.

"What the BJP has done in Delhi is not an exception. It is increasingly becoming the rule," she wrote, enumerating how the Centre is "creating problems for elected governments".

The list included "misuse" of the office of the governor, central agencies like the CBI and Enforcement Directorate, "withholding" states' funds, "disbanding" bodies like the National Development Council and Planning Commission, using money power to topple non-BJP governments, privatisation of "nation's assets" and "deterioration" in the relations between the states and the Centre.

"I strongly believe that the time has come for a united and effective struggle against BJP's attacks on democracy and the constitution... As the chairperson of TMC, I shall work wholeheartedly with you and all other like-minded parties in this battle," she wrote.

Besides Sonia Gandhi, the letter was sent to Nationalist Congress Party chief Sharad Pawar, DMK's Stalin, Shiv Sena chief Uddhav Thackeray, YSR Congress chief Jagan Mohan Reddy, BJD chief Naveen Patnaik, Telangana Rashtra Samithi chief K Chandrashekar Rao, Samajwadi Party's Akhilesh Yadav, Rashtriya Janata Dal's Tejaswi Yadav, Aam Aadmi Party chief Arvind Kejriwal and CPIML's Dipankar Bhattacharya.

In West Bengal, several thousand paramilitary troops were deployed to Nandigram, an Election Commission observer added.

Dozens of people have died in political violence in recent months.

On Tuesday, Indian cricketer turned BJP candidate Ashok Dinda blamed political opponents for a brick thrown at his car. Dinda said he had to flee from dozens of opponents and suffered injuries, with authorities tightening security around the former bowler after the incident.

And Mamata herself yesterday alleged that her car was attacked once again in Nandigram a day ago, adding that she had photographs and videos as proof and would take up the issue after the elections.

"How dare they attack my car! I kept mum only because the elections are on. Or else I would have seen how big they have grown," Mamata said at a rally in Goghat, around 90 km from Kolkata.

A visibly agitated Mamata said of her attackers, "I will see which 'gaddar' (traitor) shelters you. Where will you go? To Delhi, Bihar, Rajasthan or UP. I will drag you back here (West Bengal)."

According to news agency PTI, Mamata Banerjee was heckled on Tuesday allegedly by opposition party supporters when she visited a Trinamool Congress worker, undergoing treatment after being beaten up.

Her car was then chased by people waving placards, who Trinamool members said had attacked the vehicle, PTI reported.

Polls are being held in West Bengal over eight phases amid tight security and conclude on April 29.

Results will be announced on May 2, alongside several other state and territory elections in Assam, Kerala, Tamil Nadu and Puducherry.

Facebook has meanwhile promised to combat hate speech and misinformation across several fiercely contested states.

The social media giant said that it was working to reduce the risk of "problematic content going viral in these states and potentially inciting violence" in the elections.