The recently freed French-Colombian politician, Ingrid Betancourt, has urged her former captors, the Marxist Farc rebels, to release all hostages.
Ms Betancourt was leading a rally in the French capital, Paris - one in a series of global demonstrations calling for an end to kidnapping and for peace.
Hundreds of thousands took to the streets in Colombia, some holding photographs of missing loved ones.
The Farc has waged a 44-year civil war there and still holds 700 captives.
Up to 2,000 more people are believed to be held by the ELN (National Liberation Army), another left-wing rebel group, in remote jungle and mountain camps.
In Paris, thousands gathered near the Eiffel Tower to hear Ms Betancourt, who was freed in a daring military rescue earlier this month after six years in captivity.
She read out a list of names of those still held by the Farc (Revolutionary Armed Forces of Colombia) and called for their release.
"We want freedom for everyone," Ms Betancourt said in Spanish, amid applause and chants from the crowd of "Libertad", or Freedom.
Her speech was also broadcast in Colombia, where independence day celebrations became a mass national appeal for an end to hostage-taking and for peace moves between the government and the Farc.
Colombian pop star Shakira opened events in the Amazonian town of Leticia by singing the national anthem, flanked by President Alvaro Uribe and visiting dignitaries.
Mr Uribe pledged to work for the release of all hostages. He offered "a commitment to those who have lost their freedom so that they may regain it, a message of commitment to the new generations so that the homeland will allow them to live happily".
Marches took place in most of the country's more than 1,000 municipalities, with the biggest turnout in the capital, Bogota.
The BBC's Jeremy McDermott, who is in the Colombian capital, described huge crowds of people dressed in white t-shirts - the colour of peace - shouting "Libertad! Libertad!".
But he says the question is whether the rebels of the Farc group are going to listen.
A previous demonstration in February this year saw almost a million people take to the streets in Bogota alone.
After the successful rescue of 15 hostages earlier this month from the Farc, the best-known of whom was Ms Betancourt, the turnout was expected to be even greater, although there are no official figures.
Our correspondent says the Farc appear to be impervious to cries for an end to kidnapping, let alone an end to the 44-year civil conflict.
Earlier this week, they kidnapped 10 people travelling down the Atrato River in the western province of Choco.
While being badly hit by government offensives and a series of recent setbacks, there has been no softening of their position. Yet even the Farc will have to pay attention to not just Colombia but cries from more than 40 countries to end the kidnapping and violence, our correspondent says.