UN-backed reconciliation talks have begun in Yemen aimed at drafting a new constitution and preparing for full democratic elections in February 2014.
More than 500 representatives of various political groups are taking part in the discussions in Sanaa, which are expected to last six months.
Hardline secessionists from the south of Yemen are boycotting the talks.
The dialogue is part of a deal that saw former President Ali Abdullah Saleh stand down in 2011 after an uprising.
Mr Saleh handed over power to his deputy, Abdrabbuh Mansour Hadi, who became president in February 2012 after an election in which he stood unopposed.
South 'biggest challenge'
The national dialogue conference was originally scheduled to start in mid-November, but was delayed mainly due to the refusal of factions in al-Hiraak al-Janoubi (the Southern Movement) to attend.
Hiraak is a coalition of groups which champion southern independence.
People in the former South Yemen have long complained of political and economic marginalisation by the central government in Sanaa since unification with the north in 1990.
Opening the talks under tight security at Sanaa's presidential palace, President Hadi said resolving the grievances of the south was the main challenge of negotiations.
But he also warned that any attempt to use force to put pressure on the talks would end in failure.
"Any attempt to impose a vision to deal with this [southern] issue by force will lead to big failure and big dangers," he said.
He added that the talks would form the basis for a "new, unified, safe and free Yemen."
The president is himself a southerner, but led the brief military campaign against southern secessionists in the 1994 civil war.
Meanwhile, the UN envoy to Yemen Jamal Benomar said the dialogue provided an opportunity to address "legitimate injustices" in the south that "need to be resolved".
Most Hiraak factions agreed to take part in the dialogue following pleas from President Hadi and pressure from the UN, which threatened sanctions against any party impeding the talks.
The central government also sought to improve conditions in the south, restoring property confiscated from locals and rehiring sacked state employees.
However, a hardline faction led by the former president of South Yemen Ali Salem al-Baid - who led the secessionist rebellion in 1994 - insisted that it wanted negotiations only on southern independence.
Thousands of supporters took to the streets of the southern port city of Aden on Sunday to protest against the dialogue conference. Many carried the former flag of South Yemen and banners saying: "No dialogue under occupation. Independence is our choice".
On Monday, a Hiraak activist told the AFP news agency that a protester had been shot dead by police in the southern town of Tarim.
The Nobel Peace Prize-winning activist, Tawakul Karman, is also boycotting the dialogue in protest at the presence of officials who served under Mr Saleh during his 33 years in power.
"I will not participate in the dialogue, due to the obvious imbalance in the representation of the youths, women and civil society groups and the participation of people who have the blood of the revolution youth on their hands," she told AFP.
Sources within the former ruling General People's Congress (GPC) party said Mr Saleh would not be representing it at the conference despite still being its leader.
Ms Karman also criticised the national unity government for not having restructured the security forces, released detainees and investigated the killing of opposition protesters during the uprising, as required by the UN-backed power-sharing agreement.
The powerful tribal chief Hamid al-Ahmar, who heads the Sunni Islamist Islah (Reform) party, is also reportedly boycotting the dialogue personally in protest at the decision to give Zaidi Shia rebels in northern Yemen, known as Houthis, most seats representing the province of Saada.