* Hunger-striking opposition figure moved to hospital
* Protesters demanding his release clash with riot police (Adds U.S. State Department comment, paragraphs 8-9)
DUBAI, April 6 (Reuters) - A jailed Bahraini activist who has been on hunger strike for 58 days was moved to a hospital and fed intravenously on Friday after his health deteriorated sharply, his lawyer said.
Protesters clashed with police at a gathering of more than 5,000 demonstrators in the north of the capital Manama to demand the release of Abdulhadi al-Khawaja, one of 14 men in prison for leading "Arab Spring" pro-democracy protests last year.
"Rioters broke from a group of demonstrators on Budaiya Rd and hurled Molotov cocktails, iron rods and rocks at police," the Interior Ministry said on its Twitter account.
Khawaja's lawyer, Mohammed al-Jishi, said Khawaja had been moved from an Interior Ministry clinic to a military hospital which was better equipped. The authorities had said on Wednesday he had been moved to the Interior Ministry clinic after losing 10 kg (22 pounds).
"His condition has worsened. ... His blood pressure is down, and he is getting an IV (intravenous) drip," Jishi told Reuters by telephone.
There was no immediate report on state media about the hospital transfer. An email requesting comments from the Information Affairs Authority was not immediately answered.
His daughter, rights activist Zainab al-Khawaja, was detained during a protest on Thursday in front of the Interior Ministry. The ministry said in a statement she had "assaulted a public employee."
In Washington, the State Department said it was aware of Zainab al-Khawaja's arrest, was trying to learn more about it and urged the government to treat all detainees humanely.
"We call on the government of Bahrain to ensure appropriate treatment of detainees in accordance with universal human rights and due process," said State Department spokesman Patrick Ventrell.
Rights groups such as Human Rights Watch and Amnesty International say Khawaja, who also has Danish nationality, and the other men are prisoners of conscience and should be freed. An appeal hearing into his jail sentence began this week.
Several activists expressed concern in posts on social website Twitter about Khawaja after he was taken to a military hospital for one day last week.
The Western-allied Gulf Arab state is ruled by the Sunni Muslim Al Khalifa family but has a Shi'ite majority whose political leaders have demanded more democracy. A Shi'ite-led pro-democracy uprising was crushed last year, but protests in Shi'ite areas remain frequent and often end in violence.
Khawaja's family have identified him as case no. 8 in a report last November by a commission that described abuse of detainees without naming them.
According to the report by the Bahrain Independent Commission of Inquiry (BICI), which Manama formed under international pressure to investigate the unrest, the detainee underwent surgery on his jaw after he was beaten up upon arrest.
His abuse resumed eight days later - including beatings on the soles of his feet and being sodomized with a stick, the report said. He went on a hunger strike to stop the torture.
Bahrain's government has acknowledged the report's finding that some detainees died under torture. It says it is implementing reforms of its security forces and detention facilities that will prevent future abuses.
Almost daily protests have been held in recent weeks against plans for the small island kingdom to host the Formula One Grand Prix on April 20-22. Former world champion Damon Hill has called on Formula One bosses to reconsider the plan and warned the sport's image could suffer otherwise. (Reporting by Firouz Sedarat; Editing by Peter Graff and Todd Eastham)