A human rights watchdog warned on Tuesday that Bahraini activist Abdulhadi al-Khawaja, who was on the 55th day of a hunger strike, could die in jail.
"It is impossible to imagine that the Bahrain Grand Prix will go ahead if Abdulhadi al-Khawaja dies on hunger strike in prison," said Mary Lawlor of Front Line Defenders.
"The Bahraini authorities clearly want to present an image of business as usual but their seeming indifference to the plight of Abdulhadi, who has reached his 55th day without food, risks tragic consequences," she added in a statement.
A team of Front Line Defenders wound up its three-day visit to Bahrain on Tuesday.
But they were unable to meet Khawaja, who was condemned with opposition activists to life in jail over an alleged plot to topple the Sunni monarchy during a month-long protest that was brutally crushed in mid-March 2011.
He began his hunger strike on the night of February 8-9.
"I was able to speak briefly with him on the phone and ask him to take some food," said Lawlor, adding that "he insisted that he would continue with his hunger strike until freedom or death."
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"Unfortunately, I know just how determined he is," she said.
On Tuesday evening, police fired teargas to disperse dozens of protesters gathered outside the US embassy in Manama who were calling on the United States to pressure its ally Bahrain to free Khawaja, according to witnesses.
The police prevented the protesters from reaching the embassy compound and arrested two of them, the witnesses said.
Khadija al-Moussawi, the activist's wife, said Monday on Twitter that her husband told her over the phone that he decided to refuse taking glucose "because the situation has gone worse in our beloved homeland."
She said on Tuesday that he has two doctors watching over him and has been transferred to Al-Qalaa prison because the medical centre at Jaw prison was not equipped to handle a feared collapse.
Front Line Defenders said Khawaja, who is also a Danish citizen, has shed 25 percent of his body weight, saying this means he is "at risk of organ failure."
Front Line Defenders has called on the Bahraini authorities to allow him to travel to Denmark for medical treatment.
Bahraini activists have launched a campaign on social networks to push Formula One to cancel the Bahrain round of the world championship, slated for April 20-22 at the Sakhir circuit, southwest of Manama.
Last year's round was cancelled over the unrest which resulted in 35 deaths between mid-February and mid-March 2011, according to an independent probe.