Turkey’s often divided opposition parties have come together to choose a single candidate to face President Recep Tayyip Erdogan in May’s election.
Kemal Kilicdaroglu leads the main secular opposition party, the centre-left Republican People’s Party (CHP).
Polls suggest a tight race in a country highly polarised after two decades of Mr Erdogan’s authoritarian rule.
Economic crisis and errors during last month’s earthquake may make him more vulnerable than in previous elections.
A huge crowd of supporters cheered Mr Kilicdaroglu, a former civil servant, as he was chosen by a six-party opposition alliance.
The quiet-spoken 74 year old offers a radically different vision in both substance and style to the fiery, charismatic Mr Erdogan’s.
However, some of Mr Kilicdaroglu’s allies fear he lacks popular appeal.
He promised his supporters that he would govern Turkey through consensus and consultation.
“Our table is the table of peace,” he said, quoted by Reuters news agency. “Our only goal is to take the country to days of prosperity, peace and joy.”
He also said he would return the country to a parliamentary system. Mr Erdogan oversaw a transition to a presidential system in 2018, gaining sweeping powers.
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The CHP was created by modern Turkey’s founder, Mustafa Kemal Ataturk, and is the country’s oldest political party, though it has been out of power centrally since the 1990s.
However, Mr Kilicdaroglu has broadened its appeal by embracing minority groups and has formed alliances with right-wing parties.
He has also shown himself willing to challenge Mr Erdogan, a leader who has become increasingly intolerant of criticism.
In February’s earthquake, in which more than 45,000 people were killed in south-eastern Turkey, Mr Kilicdaroglu led attacks on the government, which he said had allowed corruption and poor building standards.