Representatives of the family said they had not received any ransom demands.

Mr Zeidan was briefly kidnapped from a hotel by a militia group in 2013, leading some to speculate that his abduction was a settling of old scores.

"There has been a powerful smear campaign against Mr Zedan as well as unclear allegations of corruption or squandering of public money," said Saleh el-Marghani, who served as justice minister in Mr Zeidan's cabinet. "The presence of Zeidan in Tripoli may have been too much for the uncontrollable militias to stomach."

Tim Eaton, an expert on Libya at Chatham House, said: "The timing of Zeidan’s visit may be linked with a potential run at the presidency in elections which have been mooted for March next year, although it is far from certain that they will take place."

Zeidan Zeidan, the former prime minister's son, said he had "no idea" why the brigade might detain his father. "This trip was a fact finding mission to work out how to get the country out of this mess. He wasn't saying 'I'm here to reclaim my throne,' or something," he said.