Tensions have been high in the Indian state of West Bengal after a top political aide from Narendra Modi’s party was shot dead in the street and hundreds were arrested as violence broke out following elections this week.

The prime minister’s Bharatiya Janata party (BJP) claimed victory in the West Bengal elections on Monday, defeating Trinamool Congress (TMC), which had ruled over the state legislature for 15 years.

Turmoil rocked the state after the results were announced. Figures from TMC, including its leader, Mamata Banerjee, said there had been widespread irregularities in the vote and accused the BJP of “looting” the election. Banerjee said she would refuse to resign, stating that she had “not been defeated”.

Late on Wednesday evening, Chandranath Rath, an assistant to the head of the BJP in West Bengal, Suvendu Adhikari, was shot dead in the streets of the state capital, Kolkata, by gunmen on a motorcycle. He was pronounced dead at the hospital.

Speaking to reporters, Adhikari, who is the frontrunner to become the state’s next chief minister, said the BJP was “shocked, pained and hurt” by the incident and alleged that the killing had taken place “because I defeated Mamata”.

He said police believed the killing was a “pre-planned and cold-blooded murder that was carried out after a recce for two to three days”.

TMC denied any involvement in the killing and called for an independent investigation. “Violence and political killings have no place in a democracy and the guilty must be held accountable at the earliest,” it said.

Rath was one of three people killed in violence that erupted across West Bengal from Monday. The state’s police chief, Siddh Nath Gupta, said that more than 200 criminal cases has been registered and 433 people had been arrested for involvement in post-poll violence.

TMC workers accused the BJP of setting fire to their party offices and bulldozing one in Kolkata. The BJP has denied the allegations.

Post-election violence is not uncommon in West Bengal, and goes back decades to when the state was under communist rule. However, the recent election has proven to be particularly controversial, after the government carried out a special revision of the electoral roll to purge “illegal” voters. That process resulted in millions of people – the majority of them Muslims or from minority groups – losing their right to vote just before the election.

Over the course of voting in April, India’s election commission ordered an unprecedented number of police and paramilitary to be deployed to the state, in the name of ensuring security. That deployment will remain in place for the next 60 days.

The BJP’s win in West Bengal was seen as a significant political coup for the Hindu nationalist party, securing its control over the east of the country and ensuring it now governed more than 70% of the country at state level.

Banerjee’s refusal to step aside threatens to cause an unprecedented constitutional crisis. On Thursday night, the West Bengal governor unilaterally dissolved her government and cabinet. TMC confirmed they would challenge the election results in the supreme court.

Despite TMC’s objections, Adhikari said the new West Bengal state government, including the next chief minister, would be sworn in by Saturday.