Greenpeace and other activists have long campaigned against the project to build an eight-lane motorway over the top of a tree-lined slope in southern Moscow that contains radioactive waste buried in the Soviet era.

"Works are beginning next to the Moscow Polymetals Plant," Greenpeace Russia said in a statement, referring to the plant that originally dumped the waste.

The former top-secret facility produced the radioactive element thorium for nuclear reactors until the 1970s.

As construction equipment arrived, dozens of police cordoned off the slope that descends to the Moskva River, activists said.

An excavator dug a hole in the ground, while workers uprooted trees and removed a fence around the plant even though the builders lacked the necessary permits to begin work, Greenpeace said.

Using a loud-hailer, police asked several dozen activists and other people to disperse, an AFP journalist saw.

Sergei Vlasov, a local councillor and activist, said police made no arrests on Wednesday and allowed campaigners, who have been monitoring the site round-the-clock from a minivan, to remain there.

He said, however, the activists had already registered higher than usual radiation at the site.

"We have registered 0.4 microsieverts," while the permitted level in Moscow is 0.3," Vlasov told AFP, adding that he expected those levels to increase in the future.

"When large-scale work begins, all this crap will be in the air," he said.

Galina Rozvadovskaya, who lives near the site, said she came as soon as she learnt of the start of the construction work.

"Our task is to stop this lawlessness," Rozvadovskaya told AFP. "What do we want? For them to conduct a proper survey of this burial site."

Moscow Mayor Sergei Sobyanin is keen to redevelop post-industrial wasteland and insists there are only "insignificant traces of contamination" on the road's route.

Activists say, however, dangerous radioactive particles could be spread around and end up in people's lungs. Citing a state report, Greenpeace says the site contains at least 60,000 tonnes of radioactive waste.

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