A political showdown is taking place in the Indian state of West Bengal as the chief minister, Mamata Banerjee – India’s most powerful female politician – has dramatically refused to resign after she lost elections to the prime minister’s party this week.

On Monday, Narendra Modi’s Bharatiya Janata party (BJP) won an overwhelming victory in the state elections in West Bengal, where Banerjee and her Trinamool Congress party (TMC) have been in power for 15 years.

But in a dramatic press conference on Tuesday night, Banerjee accused the BJP of “forcefully capturing” the elections and made it clear she had no intention of stepping down, paving the way for a constitutional crisis in the state.

“Why should I step down? We have not lost,” she said. “The mandate has been looted. Where does the question of resignation arise?” She added that TMC were “defeated not by public mandate but by conspiracy”.

Under India’s constitution, Banerjee, 71, cannot legally remain as chief minister given TMC’s loss in the election. In a statement, the governor of West Bengal said that if Banerjee did not step aside voluntarily they would send the police to “evict her” from her office. The case could also go to the supreme court.

The BJP already moved to ban advisers appointed by Banerjee from entering their offices. The BJP’s national spokesperson, Sambit Patra, called Banerjee’s refusal to step down “constitutional blasphemy”.

“What Mamata Banerjee has said and done today is deeply unfortunate. This is an attack on a longstanding democratic convention,” Patra said. “It is not an attack on the BJP, but an attack on democracy and the constitution.”

Banerjee, referred to by supporters as a fire goddess and didi, meaning older sister, built a reputation over decades as a tough, street-fighting politician as she led TMC to defeat the Communist party, which had ruled West Bengal with terror for more than 30 years.

Now, without power in the West Bengal state legislature, TMC and Banerjee have been left afloat and their position as a formidable opposition force to counter the BJP and Modi is under threat.

The BJP’s triumph in West Bengal, one of the largest and most politically important states, had been a longstanding goal for the party and was seen as one of the last frontiers in their complete dominance of India’s political landscape. The BJP won a historic 207 seats out of 294, while TMC was reduced to 80.

Banerjee alleged Modi and his home minister, Amit Shah, were “directly interfering” in the West Bengal elections, and that the chief of the election commission, who was appointed by the Modi government, was the “villain of this election”.

With the BJP now controlling 21 out of 28 states, Banerjee warned of the BJP asserting “one party rule” over the country and said she would be consulting other opposition leaders.

One of the opposition figureheads backing Banerjee’s decision to refuse to quit was Sanjay Raut, the parliamentary chair of Shiv Sena (UBT).

Raut said the election commission had become “slaves” to the Modi government and it was necessary for opposition parties to unite against the “dictatorship of the Centre and partisan behaviour of the election commission”.