Congress general secretary Priyanka Gandhi Vadra asked Prime Minister Narendra Modi on Tuesday why the person behind the killing of four farmers in Lakhimpur Kheri has not been arrested yet while she has been under "detention for the past 28 hours" without any order or FIR.

She was detained in Sitapur on Monday on her way to Lakhimpur Kheri, where eight people died and several others were injured on Sunday in the bloodiest clash since the farmers' protest over the Centre's agri laws began last year.

Priyanka Gandhi on Tuesday shared a video on Twitter, which showed a group of protesting farmers being mowed down by an SUV.

"@narendramodi ji your government has detained me for the past 28 hours without any order or FIR. Why the person who ran over farmers has not been arrested yet?" she said in a tweet in Hindi.

Congress media and communication vice chairman Pankaj Srivastava said the party general secretary, who was detained at 4.30 am on Monday, has been in detention for the past 28 hours.

"She is not being allowed to meet her lawyers and the administration is not telling her reasons for her detention," he said.

Besides Priyanka Gandhi, state Congress president Ajay Kumar Lallu, national secretary Dheeraj Gurjar, Youth Congress national president BV Srinivas, party MLC Deepak Singh are also under detention, Srivastava said.

He said there is anger among Congress workers due to the illegal detention of party leaders.

"The prime minister is coming for a celebration in Lucknow while the farmers of Lakhimpur are waiting for justice," he added.

Four of the dead were farmers, allegedly knocked down by vehicles driven by BJP workers travelling to welcome Uttar Pradesh Deputy Chief Minister Keshav Prasad Maurya to an event in the area.

The others were BJP workers and their driver, allegedly pulled out of the vehicles and then lynched by the protesters.

The UP Police has lodged a case against Union minister Ajay Mishra's son but no arrest has been made so far.