One person was killed when activists of the Samajwadi Party clashed with those from the ruling Bahujan Samaj Party in Jamo block of Gauriganj assembly constituency, that on Wednesday went to the polls along with Haidergarh in Uttar Pradesh.
BSP workers also clashed with supporters of its coalition partner, the BJP.
The bypoll caused tension in Gauriganj and Haidergarh with both places witnessing sporadic violence.
A senior home department official insisted that the violence occurred after the polling had concluded. The police had to fire in the air to bring the situation under control, he said.
He also ruled out the possibility of SP worker Abdul Rahim having fallen to a police bullet.
But SP state chief Ram Saran Das is not convinced. "Abdul Rahim was killed by the police that has been simply carrying out the brief of Chief Minister Mayawati, who is out to terrorise our partymen," he said.
Earlier, BJP leader and former Union minister Sanjay Singh is believed to have got into a fracas with two BSP ministers who were found moving around Gauriganj constituency in violation of the Election Commission's directive banning the entry of ministers.
On Tuesday, Sanjay Singh had objected to the 'gross misuse of government machinery by the BSP for the bye-election'.