Human Rights Watch said on Wednesday that police had arrested 20 opposition activists in towns near Bahrain's Formula One circuit in a sign of rising political tension before the Grand Prix on April 21.
The Bahrain government denied any arrests had taken place.
The Gulf Arab state, where the U.S. Navy's Fifth Fleet is based, has been hit by unrest since pro-democracy protests broke out in early 2011, putting it in the frontline of the region-wide tussle between Shi'ite Muslim Iran and Sunni Arab states such as Saudi Arabia.
Watched by millions around the world, the Grand Prix is the biggest sporting event hosted by the U.S.-allied country and the government is hoping for a big turnout at this year's event despite continuing violent unrest.
An HRW statement said the detentions were made without a warrant and with the apparent intention of preventing a repeat of protests during last year's race, which went ahead against a backdrop of burning tyres and riot police firing teargas at petrol-bomb throwing protesters in Shi'ite Muslim villages.
But Information Minister Sameera Rajab told Reuters: "We discredit any news of such arrests in recent days or even months,"
Nobody could be arrested without a warrant, he said.
"This doesn't happen in Bahrain. If there is any action against peace and security, it must be dealt with according to law."
The race at the Sakhir desert circuit was cancelled in 2011 when the protests were crushed and at least 35 people were killed. Activists put the tally far higher.
Almost daily demonstrations have taken place in Bahrain since the end of martial law in June 2011, often ending in confrontations as youths throw stones or petrol bombs and police fire birdshot pellets and tear gas.
The Shi'ite majority complains of entrenched discrimination - a charge denied by the government - and their loyalty has been questioned by members of Bahrain's Sunni ruling family, bound by historical and marriage ties to that of Riyadh.
Rights Watch, quoting local sources, said that in about 30 raids since April 1, masked police officers in plain clothes had targeted activists living near to the Formula One track who had led protests in the past.
The raids, mostly at night or around dawn, took place in Dar Khulaib, Shahrakan, Madinat, Hamad, and Karzakkan, towns close to the circuit and the roads leading to the capital Manamam.
"This latest crackdown and the way it's being carried out raises new questions about the Bahraini authorities' commitment to reform," HRW's Middle East director, Sarah Leah Whitson, said in the statement.
"These raids and detentions suggest that officials are more concerned with getting activists out of circulation for the Formula 1 race than with addressing the legitimate grievances that have led so many Bahrainis to take to the streets."
The international rights group said at least two of those detained were charged with crimes related to national security while others were charged with participating in illegal gatherings.
Formula chief Bernie Ecclestone said last week he had no concerns about the race becoming a target for anti-government protesters.
An international inquiry commission, invited by Bahrain's government, said in a report in November 2011 that 35 people had died during the uprising. The dead were mainly protesters but included five security personnel and seven foreigners. The report said five people had died from torture.
The opposition puts the death toll at more than 80.
Bahrain's opposition and government resumed reconciliation talks last month for the first time since July 2011, but little progress has been reported in several sessions of negotiations.