Chad's ruling party on Saturday endorsed President Idriss Deby Itno's bid for a sixth term in an election later this year.
Deby, a 68-year-old former army chief who has been in power for 30 years, is expected to face a joint opposition candidate in the April vote.
Despite ruling with an iron fist, Deby largely enjoys the support of the international community which views him as an essential ally in the fight against jihadists in the troubled Sahel region.
Deby hailed his nomination at a meeting of his Patriotic Salvation Movement.
"Allow me, my brothers and sisters, to tell you that it is after a mature and deep introspection, that I decided to respond favourably to this call of the people," he said.
The opposition had called for demonstrations on Saturday but they were banned by the authorities because of a "risk to public order".
Police fired tear gas to disperse one protest, and prominent opposition figure Succes Masra along with 10 others took refuge within the security perimetre around the US embassy at the site of a carpark where public access is usually banned, an embassy official said.
On Tuesday, 12 opposition parties said they would field a joint candidate against Deby in the April 11 poll and signed a deal creating an electoral coalition called Alliance Victoire (Victory Alliance).
Deby has been the leader of the central African state 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 rule, Deby has been accused of appointing relatives and cronies to key positions and 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.
Banned opposition demonstrations, arbitrary arrests and severed access to social networks raise regular objections from human rights groups, which have also accused the ruling class of endemic corruption.
- 'Lessons learned' -
The opposition manifesto signed by 12 party leaders on Tuesday sets down a joint approach for government, a code of conduct and "the criteria for appointing a single candidate on a transparent and consensual basis".
Signatories include two prominent opposition figures -- Saleh Kebzabo, the runner-up in the 2016 election with about 13 percent of the vote, and Mahamat Ahmat Alabo.
The manifesto says other opposition parties can join, although it does not set a date for when the single candidate will be named.
The alliance's coordinator, Alladoumngar Tedengarti, said "the lesson has been learned" from 30 years of elections in which Deby has been able to cruise past a fragmented opposition.
Other leaders who have yet to join include Laokein Kourayo Medar, who placed third in 2016, and Masra, whose campaign group, The Transformers, has joined with NGOs to call for protests.