Robert Mackey, New York Times

One of Bahrain’s most prominent human rights activists, Nabeel Rajab, was arrested on Thursday for posting critical remarks about the government on Twitter.

It was the third time that Mr. Rajab, the president of the Bahrain Center for Human Rights, has been detained for posting criticism of the monarchy since a bloody crackdown on pro-democracy protesters in 2011.

After reading the arrest warrant presented to him at his home on Thursday, Mr. Rajab made a defiant statement into a video camera held by his son, Adam. “My case is related to Twitter,” he said. “This is another attempt to suppress the people’s right to freely express their opinions.”

Before he was taken into custody by a group of at least 17 officers, Mr. Rajab added that he was sorry to see so many policemen gathered around his house “not to arrest a criminal, but to arrest someone who expresses his opinion.”

He was jailed in 2012 for posting a critical gibe about Bahrain’s prime minister on Twitter. Last year, he was arrested for posting messages about a former member of the nation’s security forces who had boasted about joining the Islamic State militant group in Syria. He was convicted both times, spent two years in prison, and was out on bail pending an appeal in the 2014 case.

According to Mr. Rajab’s colleague Said Yousif al-Muhafdah, the new charges relate to accusations by Mr. Rajab that Bahrain tortured inmates in Jaw Prison.

In March, Mr. Rajab shared images of a former prisoner’s wounds on Twitter.

Last week, he posted a link to an opinion article he wrote for Huffington Post on what he said was the use of excessive force to punish inmates who had taken part in a protest inside Jaw Prison.

In his Huffington Post op-ed article, Mr. Rajab wrote:

Jaw is facing a crisis. On 10 March there was a protest in the prison. A family at the visitation centre were told their son was barred from visits. There was an altercation with the inmate’s sister, where a police officer apparently hit her. The inmates in their visitation lobby were all taken back to the main prison buildings, where outrage sparked action.

Some prisoners began barricading their cells in protest. The authorities retaliated by locking the buildings from the outside and calling in reinforcements. Hundreds of police swarmed the prison. Buildings 1, 3, 4 and 6 — the prison is made up of ten — were subjected to a siege situation. The police broke through the barricades and flushed the inmates out with teargas. They marched the inmates out into the courtyards, where every one of them was beaten and humiliated by the police. The forces took shifts terrorising the inmates, passing the baton between Bahraini police and Jordanian units. The inmates were shot at with shotguns and sound grenades, aimed at their bodies. Inmates were forced to address the officers as ‘master’, beaten if they asked to be taken to the toilet (where they were given 30 seconds to relieve themselves), beaten during meals, and forced to insult their families or face more beatings.

“We have witness testimony and photographic evidence showing that human rights abuses are being carried out in Jaw,” Mr. Muhafdah of the Bahrain Center for Human Rights said on Thursday. “The violations are undeniable, yet rather than address the truth, they are calling Nabeel a liar and a criminal.”

Mr. Rajab’s arrest came just two days after Bahrain’s interior minister assured a delegation of visitors from the United States Congress that he was enacting a series of reforms to ensure the “human rights and freedoms” of citizens would be protected.