"Khalil al-Marzooq is a prisoner of conscience, imprisoned only for his vehement criticism of the government," the London-based organization said in a statement late on Wednesday.
"He must be immediately and unconditionally released," Amnesty's Middle East and North Africa deputy director Hassiba Hadj said.
The public prosecutor of the Sunni-ruled Gulf kingdom decided on Tuesday to hold Marzooq -- a senior figure in the main Shiite Al-Wefaq opposition group, in custody for 30 days pending investigation on charges of inciting terrorism.
"His arrest is yet another blow to the National Dialogue which the Bahraini authorities have been flaunting as a reason to cancel the visit of the UN expert on torture to the country," said Hadj.
Five opposition groups, including Al-Wefaq, on Wednesday suspended their participation in the national dialogue, which the government initiated to break the political deadlock in the Shiite-majority country.
"However harsh his speech towards the authorities, he should not have been arrested for expressing his views," Hadj said.
But the authorities insisted late Wednesday that Marzooq's arrest was unrelated to his political activism, and "relates to an investigation into his links to an extremist group", the radical February 14 Revolution Youth Coalition.
"This group has repeatedly incited and glorified attacks against police and civilians on a daily basis," an Information Affairs Authority statement said.