Two Venezuelan opposition leaders, Leopoldo López and Antonio Ledezma, have been re-arrested, relatives of pair say.
They were under house arrest after accusations of inciting violence during anti-government protests in 2014.
Their families said they did not know where the two had been taken.
The move comes just two days after a controversial vote for a constitutional assembly saw violence on the streets, with at least 10 people killed.
President Nicolás Maduro convened the assembly to rewrite Venezuela’s constitution amid spiralling anti-government protests on 1 May.
He argued that the move would create peace and foster dialogue by bringing together different sectors of Venezuela’s polarised society.
But the opposition accused the president of trying to rewrite the constitution in order to maximise his power and sideline the opposition-controlled legislature.
They boycotted the vote and called on Venezuelans to take to the streets in protest. The election was condemned by Latin American leaders, the European Union and the US.
The day of the election was the deadliest so far since the current wave of protest began.
US Treasury Secretary Steven Mnuchin announced on Monday that the US had imposed sanctions on President Maduro and called him a “dictator who disregards the will of the Venezuelan people”.
Days before the vote, Mr López recorded a video from his home, the first since he was transferred to house arrest on 8 July after spending more than three years in jail.
In it, he said that President Maduro was threatening democracy in Venezuela.
Mr Ledezma published a video on YouTube on Monday in which he called the constituent assembly a “fraud” and spoke of the “tyranny” Venezuela was experiencing.
Mr López was taken from his home at 12:27 local time (04:27GMT) on Tuesday, his wife, Lilian Tintori, wrote on Twitter.
A video posted showed him being taken away by members of the Venezuelan intelligence service, Sebin.
Lilian Tintori wrote that she would hold President Nicolás Maduro responsible if something were to happen to her husband.
The daughter of Mr Ledezma, Vanessa Ledezma, also posted a video of her father, wearing pyjamas, being taken away by the Sebin.
A woman can be heard shouting “They’re taking Ledezma, they’re taking Ledezma, dictatorship!”
Mr Ledezma was first arrested in February 2015, when he was mayor of Caracas, on charges of planning the overthrow of President Maduro.
He has been under house arrest since April 2015 because of health problems.
Both he and Mr López were key figures in the wave of protests which swept through Venezuela in 2014 in which 43 people from both sides of the political divide were killed.
They have played a less prominent role in the most recent protests because they have been under house arrest but their video messages still get reported and shared widely on opposition websites.