Prominent Bahrain human rights activist Nabeel Rajab has been arrested over tweets about torture practices in a prison, his wife says.
Sumaya Rajab said that more than 20 police vehicles had been sent to detain him.
The activist has served several prison sentences since setting up the Bahrain Centre for Human Rights in 2002.
Human Rights First said the arrest was an "alarming setback" to hopes for easing the nation's political crisis.
Bahrain's Sunni Muslim rulers forcibly quelled a 2011 uprising which had been triggered in large part by unrest among the Shia Muslim majority.
Sumaya Rajab said her husband was arrested at 14:15 local time (15:16 GMT).
She said: "There were a lot of police, many more than needed to arrest one man. My relatives counted more than 20 police vehicles.
"Nabeel was sleeping and my daughter woke him to say the police had come. I went to the gate and asked what they were doing. They showed me an arrest warrant that said he was being arrested for tweets he had made about torture in Jaw prison."
Mr Rajab has already been charged over another tweet he sent in September in which suggested that security institutions in Bahrain served as what he called an "ideological incubator" for jihadists.
He was released on bail and his next court appearance on that charge is schedule for 14 April.
Human Rights First said Thursday's arrest was "the latest in a string of actions that should give Washington pause as it considers whether to lift restrictions on sending arms to the kingdom".