Colombian right-wing paramilitary commander Salvatore Mancuso has begun testifying before a special tribunal.
Mancuso's testimony so far has not been made public
As he was led into the court, relatives of those killed by the paramilitaries held banners demanding justice.
Mancuso's appearance before the tribunal investigating hundreds of civilian deaths at the hands of paramilitaries is part of a peace deal.
His group is involved in drug smuggling and in a long-running conflict with state forces and left-wing rebels.
Tens of thousands of people have been killed in four decades of fighting and many more have been displaced.
Mancuso is the first high-ranking member of the United Self-Defence Forces of Colombia (AUC) to give testimony as part of the 2003 peace deal which has led to the demobilisation of thousands of paramilitary fighters.
His appearance in court has been highly anticipated in Colombia.
In a testimony expected to last several days, Mancuso is expected to confess involvement in various crimes and name politicians involved in deals with the paramilitary forces.
His testimony so far has not been made public.
In 2003, Mancuso was sentenced in absentia to 40 years in prison for his part in a 1997 massacre.
Under the peace deal reached with the government, paramilitary fighters are eligible for reduced jail terms - of no more than eight years - if they give details of their involvement in torture, killings and other crimes.
The peace deal has been criticised by human rights activists who say it amounts to an amnesty.