Seven Bahraini policemen have been wounded, three of them seriously, when a home-made bomb exploded during a protest near the capital calling for the release of an activist on a two-month hunger strike.
Protesters threw petrol bombs at riot police to lure officers into Eker, a Shia village outside the capital Manama, before the explosion was set off, an interior ministry spokesman said on Monday.
"We consider this an act of terrorism," the spokesman said of the explosion, an escalation in violence which could cast further doubt on the staging of the Formula One grand prix this month in the Gulf kingdom that hosts the US Navy's Fifth Fleet.
On Sunday, Bahrain ruled out extraditing the jailed Bahraini political activist, Abdulhadi al-Khawaja, also a Danish citizen, despite a request from Denmark to hand him over because his health was worsening after his hunger strike.
Meanwhile, UN Secretary-General Ban Ki-moon has said Bahrain should consider transferring Khawaja to Denmark for medical treatment on humanitarian grounds.
UN spokesman Martin Nesirky told reporters that "in cases where there is a hunger strike, the health and well-being of the person should be the foremost concern".
Daily protests to demand his freedom have been taking place across Bahrain, which crushed protests last year with the help of troops from Saudi Arabia and the United Arab Emirates.
Western rights groups say Khawaja and 13 other opposition figures in prison for their role in last year's protests are prisoners of conscience and should be freed.
Khawaja's lawyer said the activist had been moved to a military hospital and was being given an intravenous drip.
Zainab al-Khawaja, the daughter of Abdul Hadi al-Khawaj, told Al Jazeera that the family had "no idea" about the state of his health as they had not been allowed to call or visit him.
Earlier on Monday, his lawyer Mohammed al-Jishi told the AFP news agency: "Authorities have been refusing since yesterday all requests, made by myself and by his family, to visit or contact al-Khawaja."
Jishi said the last time he contacted Khawaja was on Saturday, a day after he was moved from the interior ministry hospital to a military hospital in Manama.
In a statement on Monday, Bahrain's interior ministry said: "Abdulhadi al-Khawaja's state of health is good."
It said he had been transferred to a military hospital "for the best medical treatment", adding that he was being handled without "political or media pressure and with respect for human rights."
Speaking to Al Jazeera, Fahad al-Binali, from Bahrain's Information Affairs Authority, said that it "must be remembered that the convictions against Abdul Hadi al-Khawaja are serious charges".
"Any person who demands reform must understand that the rule of law comes first," he said.
Protesters have also demonstrated against plans to host the grand prix. Last year's race in Bahrain was postponed, reinstated and then cancelled due to the uprising and bloody crackdown.
The governing International Automobile Federation (FIA), commercial supremo Bernie Ecclestone and Bahrain organisers have all said the April 22 race is on.
But Formula One teams headed to China on Monday for a race on April 15 still unsure whether their return trip would take in Bahrain for the following race amid the safety fears.
Team sources told Reuters news agency that some had hedged their bets by routing personnel on return flights via Abu Dhabi, Dubai or Oman with alternative reservations for the last leg of the journey back from Shanghai.