An Indian teenager who accused a legislator of the ruling Bharatiya Janata Party (BJP) of rape was fighting for her life on Monday following a car crash that killed two aunts and also injured her lawyer, raising suspicions of foul play.

The accident occurred in northern Uttar Pradesh state Sunday when a truck collided with their car.

Police said Monday they had arrested the truck driver and were investigating claims that it may have been a hit to silence the young woman.

"Our inquiry is going on and we will look into the family's allegations," Rajeev Krishna, the additional director-general of police, told reporters in Lucknow, the state capital.

The National Commission of Women, a top rights body, urged police to conduct an "absolutely free, fair and speedy" probe and said its team would soon meet the teenager's family.

The 19-year-old accused state lawmaker Kuldeep Singh Sengar of raping her at his home in 2017, but police in the notoriously lawless state initially refused to take action. 

Her father was detained by police and severely beaten in custody – allegedly by Sengar's brother – before dying from his injuries. The teenager also tried to set herself on fire outside the home of Uttar Pradesh leader Yogi Adityanath, a BJP stalwart, triggering nationwide outrage. 

Sengar, who has been in jail since last year, has denied the accusation of rape. His lawyer, Awadhesh Singh, said the case was a conspiracy to harm his political career.

"It's just an accident," he told Reuters on Monday, referring to the car crash.

However, police have lodged a case of murder against Sengar, based on the family's complaint that he was involved in causing the crash, according to a copy of the report seen by Reuters.

The teenager's family said the crash was a deliberate attempt to kill her so the rape case does not go to trial. One of the aunts was a witness in the case.

The woman's family said it feared for its safety. "This is not an accident," her mother told reporters at the hospital, adding that the family had faced threats over the rape case.

"One by one, all the witnesses are being eliminated. We're afraid for our lives," she said.

Media reports said suspicions had been raised because the truck's registration number was painted over.

India's parliament was briefly suspended Monday because of opposition protests over the case.

Priyanka Gandhi, a leader of the opposition Congress party, said the accident was "shocking" and questioned why Sengar was still a member of the BJP.  

Supporters meanwhile held a rally for the teenager in the nation's capital New Delhi late Monday, holding up banners with the words "you are not alone" in Hindi.

"As citizens, we came out to say 'this is unjust'," Yogendra Yadav, an academic and politician who heads the socio-political organisation Swaraj India, told AFP news agency.

"We are here just to tell that brave girl who has suffered so much during the last two years that this country's conscience is with you." 

The case has shone a spotlight on India's atrocious record on rape.

Nearly 40,000 cases are reported every year, according to official figures. Campaigners say the real number is higher, with victims wary of filing cases because of the social stigma and a lack of faith in investigators.