Colombian President Juan Manuel Santos said peace talks could falter if left-wing Farc rebels do not halt attacks, as he was sworn in for a second term.
"Gentlemen of the Farc: you have been warned," the president said, to a crowd of more than 2,000 guests, including foreign delegations from 73 countries.
The 62-year-old was re-elected for a second term in June.
His victory was a resounding vote of confidence for the peace process he began with the rebels in November 2012.
Colombia has suffered more than five decades of conflict which has led to more than 200,000 deaths.
President Santos reiterated his commitment to peace talks at his inauguration ceremony in Bogota on Thursday.
"Acts of peace, that's what the Colombian people ask for today... I will employ all my energy to bring peace during this mandate," he told the crowd.
The challenges facing President Santos are many.
First he will be expected to reach a final agreement with the Farc, which even though its numbers had dwindled, still has an estimated 7,500 active fighters.
Then he will have to negotiate a peace deal with the National Liberation Army (ELN), a smaller left-wing guerrilla group which commands some 2,500 rebels.
He will also have to ensure that any deal he reaches with the rebels has the support of the population.
He also faces considerable opposition from his conservative rival, Ivan Zuluaga and his mentor, former President Alvaro Uribe, a fierce critic of the peace talks.
President Santos has expressed anger at the rebels in recent days about attacks by Farc on the civilian population and on infrastructure.
But he has repeated that while talks taking place in Havana continue, the Colombian military will not drop its guard.
"We will continue fighting because the fastest way to reach the end of this conflict is not to cede even a centimetre of our territory while we continue negotiations abroad," he said.
Talks are expected to resume on 17 August when a first group of representatives speaking for the victims of the conflict will arrive in Havana.