SINGUR, India (AFP) - About 40,000 protesters on Sunday surrounded the Indian factory slated to produce the world's cheapest car, alleging land for the site was forcibly taken from local farmers, police said.

Demonstrators blocked main roads near the plant at Singur, 35 kilometres (20 miles) northwest of the state capital Kolkata, as riot police protected the factory premises, authorities said.

"Security has been tightened in and around Tata's small car project. More than 4,000 policemen have been deployed," Raj Kanojia, a senior police official of West Bengal state, told AFP.

The gates of the factory, which is still under construction, have been fenced off, Kanojia said, as an October deadline for the first 2,500-dollar Nano car to roll off the assembly line appeared under severe threat.

Kanojia put the number of protesters at 40,000, though activists said they expected as many as 200,000 to join the action in the coming days.

West Bengal opposition chief Mamata Banerjee, who called the protest, demanded that the land -- much of which has not been built on -- be returned to farmers.

"Our party will fight to the finish to get the land back," Banerjee said to thunderous applause on Sunday.

Our agitation will remain peaceful unless we are provoked.

On Friday, Ratan Tata, whose Tata Motors is India's top vehicle-maker, warned he would move the plant out of the state if the demonstrations kept up, although his company has already invested 350 million dollars in the project.

But activists at Singur say they will only call off protests if the government hands back 400 acres (160 hectares) forcibly taken from farmers, who have not accepted any compensation.

"The government acquired 997 acres of land for the project. The project needs about 600 acres," said Partha Chatterjee, a senior opposition politician.

West Bengal Chief Minister Buddhadeb Bhattacharjee, whose communist-controlled government brought the plant to the state, said the protests would fail.

"The project will be a reality," said Bhattacharjee late Saturday. "Nobody can obstruct it.

It is totally impractical to give back the land as demanded. Returning the 400 acres means scrapping the entire project.

Tensions were high across West Bengal, which has been at the forefront of recent battles over land rights in India.

The struggles have pitted the interests of farmers who say they will starve without their land against those of business and India's government, which say the country needs to industrialise rapidly.

For more than two years the Tata factory site has seen protests by activists and villagers who say many poor farmers were forced to part with their land.

Elsewhere in the state, at Nandigram village, where the state government acquired land for a petrochemical hub, police in March 2007 shot dead 14 farmers opposing the move.