Mohamed al-Ra’id is being held at a home in Tobruk after he was seized on Wednesday on his way to the airport after being sworn in, said the official in parliament’s security service.
“The man holding Mr Ra’id is demanding the transfer of his two sons, who have been convicted in a drugs case, from Misrata prison to another one in the east of the country,” the official told AFP.
Local dignitaries are mediating to secure the politician’s release, he said.
Ra’id was elected in 2014 in the eastern city of Misrata but had been boycotting the parliament until this week.
UN envoy Martin Kobler, writing on Twitter, “strongly” condemned the kidnapping.
“I call on Tobruk’s influential actors to ensure his immediate & unconditional release,” he wrote.
Ra’id “should not pay the price for his courageous decision to uphold the national interest, risking his life to rejoin” parliament, Kobler added.
The kidnapping comes at a time when UN-brokered peace efforts have faltered.
World powers have urged Libya’s warring factions to endorse a unity government agreed last week under a peace deal brokered by the United Nations.
Libya has been in political turmoil and rocked by violence since the ouster of longtime dictator Muammar Gaddafi in a 2011 revolution.
It now has two governments and parliaments, with the recognised authorities based in the east and a militia-backed authority in Tripoli.