Freeman Mbowe (C) was arrested last week along with 11 other party officials (Getty Images)
Detained Tanzania opposition leader Freeman Mbowe has been charged with terrorism-related crimes, reports say.
In a tweet his party, Chadema, also announced that its chairman was facing terrorism charges.
Dar es Salaam police chief Muliro Jumanne told the BBC that Mr Mbowe was arraigned in court on Monday on charges "that would have likely wreaked havoc in this country".
In Tanzania law, terrorism-related charges are not bailable and the politician will continue to remain in custody.
Police arrested Mr Mbowe, along with 11 other party officials, last week in the northern city of Mwanza. They were meeting to discuss a push for a new constitution, a move the government opposes.
His arrest has been linked to various charges including conspiracy to commit terrorist acts and killing government officials.
The move has however been widely criticised locally and internationally, with human rights groups including Amnesty International calling for his release.
A committee of US lawmakers has also expressed concern over the arrest and detention.
Karen Bass, the chairperson of US House Foreign Affairs Subcommittee on Africa, said recent developments had "unfortunately minimised any hope" for a more democratic society.
She said President Samia Suluhu Hassan had appeared "to be on a different path for leading Tanzania forward to a more democratic society" after taking over from President John Magufuli, who died in March.
This article was published by BBC News.