Two Venezuelan opposition leaders, Leopoldo López and Antonio Ledezma, have been re-arrested in a move condemned internationally.
They were under house arrest after accusations of inciting violence during anti-government protests in 2014.
The two have been taken to Ramo Verde military jail, family members say.
The Supreme Court, which critics say is dominated by government loyalists, said it had received intelligence reports alleging the two were trying to flee.
The court also said that it had revoked Mr Ledezma's and Mr López's house arrest because they had made political statements.
The UN's High Commissioner for Human Rights, Zeid Ra'ad al-Hussein, said he was "deeply concerned" that the pair had been taken back into custody.
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".
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!"
The European Union criticised the detentions as a step in the wrong direction and Chile's foreign minister said they "sent an abysmal signal".
The US also condemned the arrests. Republican Senator Marco Rubio noted that US Vice-President Mike Pence has spoken to Mr López last Friday, and characterised the arrests as a response to the sanctions placed on Mr Maduro.
Both Mr Ledezma 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.
- 46-year-old leader and founder of the Venezuelan opposition People's Will (Voluntad Popular) party
- Sentenced to 13 years and nine months in jail in September 2015 for incitement to violence
- Has always denied any wrongdoing, his wife heads an international campaign for his release
- Released from military prison and put under house arrest on 8 July 2017 for "health reasons"
- Published a video on YouTube on 25 July urging Venezuelans to protest against the constituent assembly and the military
- 62-year-old mayor of Caracas
- Arrested February 2015 on charges of supporting an attempted coup, which he denies
- Put under house arrest in April 2015
- Long-standing opponent of President Hugo Chávez and his successor President Nicolás Maduro
- Published a video on YouTube on 31 July calling the election for a constituent assembly a "fraud"