Chad's main opposition candidate, Saleh Kebzabo, on Monday announced he was withdrawing from the country's upcoming presidential elections, accusing veteran leader Idriss Deby Itno of using force to intimidate rivals.
In a statement, Kebzabo said his party had decided "purely and simply" to quit the race, a day after two people were killed when security forces tried to arrest another candidate, Yaya Dillo Djerou.
Deby has ruled the huge Sahel state for over 30 years and is running for a sixth term on April 11.
Kebzabo condemned what he called a "military attack" on Yaya Dillo's home.
"The climate of insecurity... will definitely overshadow the electoral campaign of candidates confronting (Deby's) Patriotic Salvation Movement (MPS)," he said.
By withdrawing from the race, said Kebzabo, he was refusing to "provide cover for a large-scale masquerade".
Kebzabo, a former journalist who in the late 1990s was a minister under Deby, has contested the presidency four times.
He came in second in the 2016 elections with 12.8 percent of the vote.
On Sunday, a gun battle erupted at Yaya Dillo's home in the capital N'Djamena when security forces came to arrest him.
He had holed up with "armed individuals" after refusing to obey two arrest warrants, filed last year for allegedly slandering the president's wife, Hinda Deby Itno, according to the government.
The government said the fatalities happened when the security forces came under fire from his home.
But in a string of posts on social media, Yaya Dillo said his home had been surrounded and his mother and several relatives had been killed.
In his final messages, he said an armoured vehicle had smashed down the door, and urged the public to "rise up."
Yaya Dillo, a former rebel chief, once served as a minister and advisor to Deby, who is also his uncle and fellow member of the Zaghawa ethnic group.
He had filed papers to contest the elections on Friday.
Deby, a 68-year-old former armed forces chief, has been Chad's ruler since December 1990, when he ousted the autocratic leader Hissene Habre.
He has been re-elected every five years since then, thanks to constitutional changes approved by a referendum in 2005 to remove limits on presidential terms.
During his long rule, Deby has been accused of authoritarianism and nepotism as well as failing to address the poverty that afflicts many of Chad's 13 million people.
Despite oil wealth, the country ranks 187th out of 189 in the UN's Human Development Index (HDI).