Solidarity activists were waiting for news on the fate of agricultural workers union Fensuagro vice-president Huber Ballesteros who was detained for "rebellion" by intelligence officers in the capital Bogota.
Agents for the right-wing government swooped at 3.30pm local time on Sunday, hours before he was scheduled to visit the British embassy for a visa to attend the annual trade union parliament.
His whereabouts were unknown today.
Mr Ballesteros, a national organiser for social justice movement the Patriotic March, had been forced to travel with guards due to repeated death threats.
He is one of the driving forces behind huge rolling strikes that have gripped Colombia in the past week.
TUC general secretary Frances O'Grady demanded his release.
She branded his detention "on the catch-all charge of 'rebellion,' used to imprison countless trade unionists without charge over the years, is a sign that little has changed in Colombia."
The timing of his arrest, "as a strike wave grows and Huber is due to travel abroad to win international support, is nothing short of predictable," Ms O'Grady said.
The protests reflect the anger over the effects of right-wing economic policies including a free trade deal with the EU that came into force on August 1.
President Juan Manuel Santos has publicly shrugged off the strikes but the police crackdown has been brutal.
Around 200 protesters have been arrested and 100 injured in the past week.
Blockades and walkouts that began on August 19 among agricultural workers have since spread to the health, transport, mining, energy and oil sectors.
Teaching unions were set to meet today to discuss joining the industrial action.
Justice for Colombia's Mariela Kohon said Mr Ballesteros's arrest was "an act of intimidation against the strikers."
She said: "What we fear is that this is the start of a witch-hunt of all the leaders of the opposition and the trade union movement."
Ms Kohon said the campaigning group would be raising the issue at "the highest level" in Britain, which is a key backer of the country's military.