Bahraini liberal opposition figure Ibrahim Sharif was charged on Monday with "inciting hatred" against the regime with his tweets, a human rights group said.
The Britain-based Bahrain Institute for Rights and Democracy (BIRD) said Sharif was questioned by the Gulf state's public prosecution about comments he tweeted.
Apart from voicing support for detained rights activists, Sharif questioned moves by the justice ministry to dissolve the secular, opposition National Democratic Action Society (Waed) of which he was a founding member, the group said.
"What remains of the decor of the democratic state?" Sharif asked in a tweet.
Amnesty International condemned the charge against Sharif, who it said had been "in and out of prison as a prisoner of conscience" over the past six years, and called for it to be dropped.
"Once again Ibrahim Sharif is being unjustly punished simply for exercising his right to freedom of expression. The charge against him is ludicrous and must be dropped immediately," the rights group said.
"The Bahraini authorities have repeatedly sought to harass and intimidate anyone who dares to speak out about human rights violations in Bahrain," said Lynn Maalouf, a deputy director at Amnesty's office in Beirut.
Sharif, a Sunni who supported Shia-led protests in a failed 2011 uprising in the Sunni-ruled kingdom, was released from jail in July 2016 after having served a year, also on a charge of inciting hatred against the regime.
He served a four-year jail term after the protests were crushed before being released under a royal amnesty in June 2015.