Authorities took Nabeel Rajab to the cardiac care clinic of the Bahrain Defense Force Hospital, said Sayed Alwadaei, the director of advocacy at the Bahrain Institute for Rights and Democracy.
Rajab's condition wasn't immediately known today. Bahraini officials did not respond to a request for comment and state media did not immediately report on the 52-year-old activist's hospitalisation.
"We raised our extreme worries about the effects isolated detention would have on Nabeel's health and we were ignored," Alwadaei said in a statement. "Nabeel never suffered heart problems before."
Rajab helped lead protests during Bahrain's 2011 demonstrations as the island's majority Shiites and others demanded more political freedom from its Sunni rulers.
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The government quashed the protests with the help of Saudi Arabia and the United Arab Emirates, though low-level unrest and attacks on police have continued.
Bahrain, a tiny island that is home to the US Navy's 5th Fleet, has launched a new crackdown on activists since April.
Late yesterday, the Interior Ministry announced a series of arrests of people who were "misusing social media" to "incite people or instigate people to abuse others." It offered no further details on the number or identifies of those arrested.
Authorities now seek to dissolve the political party in court.
Today, lawyers representing Al-Wefaq in those hearings issued a statement saying they would withdraw from the case as authorities had blocked them from entering the group's shuttered headquarters to obtain documents and prepare their defense.
Bahrain, just off Saudi Arabia's coast, faces mounting economic pressure as its oil-dependent economy suffers from depressed global crude prices.
The agency Fitch Ratings on Tuesday downgraded Bahrain's credit rating by a notch below investment grade, putting it into junk territory.
The decision by Fitch follows a similar move in February by Standards & Poor's to put Bahrain in junk territory.