Chad's main opposition candidate Saleh Kebzabo has announced his exit from the presidential election set for 11 April.
Mr Kebzabo has accused the incumbent, Idriss Déby, of using force to intimidate rivals.
President Déby to power through a military coup in 1990 and is serving his fifth term. He is a strong ally of the West in regional counter-terrorism operations.
The announcement by Mr Kebzabo follows the killing of at least two people as security forces tried to arrest another opposition leader, Yaya Dillo, at his home in the capital, N'Djamena.
"The climate of insecurity... will definitely overshadow the electoral campaign of candidates confronting (Déby's) Patriotic Salvation Movement (MPS)," he said.
Mr Kebzabo also announced the decision on Twitter: