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".
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.
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