About 3,000 Colombians are being held by kidnappers, according to the anti-abduction citizens' group Pais Libre.
Wearing white T-shirts and waving flags, thousands marched on Bogota's main plaza. Leading the march was Alvaro Uribe, the president, whose shirt said ''Unconditional freedom now!"
The demonstrations were the largest since October 1999, when an estimated five million Colombians joined in a nationwide protest against violence and kidnappings.
Protests or moments of silence were also organised abroad by Colombian embassies and consulates in France, Brazil, New York and Poland.
The protesters demanded "freedom without conditions now" for the more than 3,000 people held hostage by guerrillas, right-wing paramilitary groups and criminals across the Andean nation.
Families of hostages, non-governmental groups and the Roman Catholic church organised the protest after the Revolutionary Armed Forces of Colombia, a rebel group known as Farc, said 11 provincial legislators it had held hostage were killed in a military raid, which the government said never happened.
Impasse
The government, which has been battling the rebels for decades, accuses Farc of executing the legislators, held hostage since 2002.
The 11 were among a group of 56 hostages, including Franco-Colombian politician Ingrid Betancourt and three Americans, who Farc wants to swap for 500 of its fighters held in prison.
Some families of those kidnapped want Uribe to negotiate with Farc for a prisoner swap, but he is unwilling to withdraw troops and create the demilitarised zone that Farc wants as a condition for talks.
Uribe says such a zone would allow Farc to regroup just like it did when troops were pulled back by his predecessor.
Families say the impasse is prolonging the suffering of those being held hostage.
The protests came on the same day that Farc released a Colombian geologist, Juan Carlos Posada, who had been kidnapped in the eastern province of Choco in March, the International Committee of the Red Cross said.