The most radical of the Shiite opposition, the February 14 Youth Movement, used text messaging and social networks to urge protesters to hold fresh demonstrations to coincide with the race.
Early Sunday, activists used burning tyres and rubbish to block roads leading to the Sakhir circuit, witnesses said. An AFP photographer said the route to the circuit, though jam-packed with traffic, was later cleared.
Bahrain's interior ministry also said in a statement that "all roads and routes leading to the (Sakhir) circuit were safe and open."
Security was tight in the island kingdom for the highly controversial Grand Prix, with police patrolling the capital Manama and the roads leading to the race track.
Police helicopters flew low over Manama and armoured vehicles were deployed across the city, witnesses reported.
The paddock, meanwhile, filled early with teams, sponsors, supporters and media anxious to avoid traffic hold-ups or other problems.
Several thousand people, who each paid 155 Bahraini dinars (310 euros) for the final day or 180 days for the whole three-day event, were gathered in the vast space set aside for spectators.
Men in Arab dishdashas stood along Westerners and women in shorts milling around the souvenir stands at the desert circuit.
With the race more than half way through, cameras were focused on the cars rather than the crowd.
King Hamad earlier announced that he would attend the event, in an apparent effort to allay security concerns even as clashes broke out overnight in the kingdom's Shiite villages.
Bahrain insists the unrest is isolated, but tensions have mounted as majority Shiites protest against their Sunni rulers, taking advantage of the media spotlight on one of the Gulf state's showcase events.
In a statement issued early on Sunday, King Hamad pledged he was committed to reform efforts in the kingdom.
"I also want to make clear my personal commitment to reform and reconciliation in our great country. The door is always open for sincere dialogue amongst all our people," said the monarch.
His announcement came as police fired tear gas and stun grenades at Shiite protesters who responded by hurling rocks and fire bombs while chanting "Down with Hamad," witnesses told AFP.
In protests that lasted through the night and into early Sunday, demonstrators also called for the release of prominent Shiite activist Abdulhadi al-Khawaja, on hunger strike since early February and whose deteriorating health has raised fears he may die in prison.
In a message posted on micro-blogging website Twitter, Bahrain's interior ministry said Khawaja was in "good health" and would meet Denmark's ambassador today." Khawaja is a dual citizen of Bahrain and Denmark.
Also on Sunday, local rights groups reported that authorities the previous day arrested two prominent activists, including Khawaja's daughter Zeinab.
The Youth Society for Human Rights said Zeinab would face interrogation by authorities on Sunday for taking part in demonstrations, but it was not clear if she was still being detained.
Another activist, Mohammed Hasan, 26, was beaten by police with the barrel of a gun, another rights group said.
Rights watchdogs have repeatedly called for Khawaja's release and denounced the government's reforms as mostly hollow.
They also warned that the continued political stalemate could set Bahrain on a path to more violent confrontation.
Last week, the International Crisis Group think tank issued a conflict risk alert for Bahrain, arguing that the decision to stage the F1 race and Khawaja's imprisonment were both ticking "time bombs" in the kingdom's 14-month uprising.
In some Shiite villages, including Malkiya, Karzakan, Sadad and Damistan, protesters carried banners that read "No to the formula of blood," a key campaign slogan of the February 14 movement.
In Manama, meanwhile, police prevented planned protests at a central market, residents and witnesses said.
On Saturday a protester was found dead in the Shiite village of Shakhura.
The opposition Al-Wefaq movement said security forces on Friday night "attacked peaceful protesters, brutally beating some of them with various tools and weapons" in Shakhura
The interior ministry said it was probing the death of 36-year-old Salah Abbas Habib.
The violence has centred mostly in and around Manama's neighbouring Shiite villages but on Wednesday, four mechanics from the Force India team were caught in a traffic jam as a petrol bomb exploded nearby.
Two team members later left the country and the India Force crew cancelled Friday afternoon's practise run.