Romanian police clashed with villagers on Wednesday as they tried in vain to force them off a field they have occupied for a third day to prevent US energy giant Chevron from drilling for shale gas.

Hundreds of protesters blocked access to the site at Silistea in eastern Romania where Chevron plans to drill an exploration well, lying down in the mud and holding hands to form a human chain.

Some 250 anti-riot police engaged in an hours-long stand-off with the protesters, with skirmishes as they physically tried to force them off, but the demonstrators pushed their way back onto the field.

The group of protesters, some of whom have been sleeping at the site since Monday, had grown to about 500 on Wednesday, preventing Chevron bulldozers and excavators from accessing the site.

Orthodox priests also joined the protest.

Many of the villagers in the rural region arrived on horse carts, some brought their children who held up signs reading: "Stop Chevron!", while an elderly woman leaned on her cane beside them.

They are afraid of the environmental and health impact of the highly controversial method used for shale gas drilling, called hydraulic fracturing or 'fracking'.

The technique consists of pumping water and chemicals at high pressure into deep rock formations to free oil and gas.

Environmentalists say fracking may contaminate ground water and even cause small earthquakes.

Chevron has permits to explore for shale gas in three villages in this part of eastern Romania as well as on Romania's Black Sea coast.

"Chevron is committed to building constructive and positive relationships with the communities where we operate and will continue our dialogue with the public, local communities and authorities on its projects," the company said in a statement to AFP.

"Our priority is to conduct ... activities in a safe and environmentally responsible manner consistent with the permits under which we operate," it added.

Also Wednesday, more than 2,000 people staged a protest in the capital Bucharest, shouting "no to shale gas".

Romania's ruling centre-left coalition has been defending shale gas exploration after fighting it when it was in the opposition.