* Island kingdom hit by protests, sectarian tension
* No repeat of last year's mass demonstrations
* Opposition report arrests, denied by government
DUBAI, April 23 (Reuters) - Bahrain security forces thwarted attacks and found caches of weapons including 1,000 petrol bombs in the run-up to last weekend's Formula One race, state media said as protests and sectarian tensions continued to simmer in the island kingdom.
Bahrain did not see a repeat of the mass demonstrations that overshadowed last year's race - though young men armed with rocks did clash with police in outlying villages, as they have done regularly since unrest erupted in early 2011.
Protests in the Gulf Arab country, a Western ally that hosts the U.S. Navy's Fifth Fleet, broke out two years ago, with the Shi'ite-led opposition drawing thousands of demonstrators demanding democratic reforms from the Sunni-led government.
Witnesses at the Sakhir desert race circuit, about 30 km (20 miles) southwest of the capital Manama, said there was no sign of unrest in the immediate vicinity but protesters blocked several roads in villages near the capital.
The opposition said the race was being used as a public relations stunt, but the government insisted it was a pure sporting event that should not be 'politicised'.
Security sweeps ahead of this year's contest "thwarted a number of terrorist plots that aimed to affect normal life ... harm the reputation of the nation and commit terrorist acts against policemen", Bahrain's chief of public security, Major-General Tariq Al-Hassan, said according to the BNA agency.
Security forces had found several weapon caches holding 1,000 petrol bombs, 19 mock bombs, bullets and homemade guns, he added.
Hassan said security forces had also handled several incidents of rioting, including "acts of chaos and destruction" inside an industrial secondary school by students who had also blocked nearby roads and attacked cars, pedestrians and policemen, according to BNA.
"Police at no point in time raided the school or attacked it," Hassan said.
Sayed Yousif al-Muhafdha of the Bahrain Centre for Human rights had said on Sunday police had fired teargas at a secondary school in the city where students had been demonstrating.
On Tuesday, Muhafdha added security forces had arrested up to 50 "pro-democracy activists" in the days preceding the race.
Amnesty International said human rights activists reported dozens of protesters were arrested ahead of the race and Human Rights Watch said on April 10 that 20 opposition activists had been arrested in towns near the circuit.
The government has denied those arrests have taken place. It has said several people accused of stealing and burning cars had been detailed.
Widespread unrest forced the cancellation of the 2011 Formula One race and although the event went ahead in 2012, it was overshadowed by violent protests in the country. (Writing by Yara Bayoumy; Editing by Sami Aboudi and Andrew Heavens)