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A man who flew a drone carrying radioactive sand on to the roof of the Japanese prime minister's office has been arrested, Tokyo police say.

Yasuo Yamamoto, 40, was protesting over the Japanese government's nuclear energy policy. He turned himself in late on Friday, police said.

No-one was hurt. Prime Minister Shinzo Abe was out of the country.

The drone landing triggered a security alert and raised fears of extremists using drones to carry out attacks.

The small amount of sand in the drone - which was equipped with a small camera - carried traces of radiation.

Police said the radioactive material was likely to be caesium but the levels were too low to be harmful to human health.

Japan does not yet regulate low-altitude drone flights except around airports. However, officials are looking into changing the law as the remote-controlled devices become more popular.

Japan shut down all 48 of its nuclear power plants after the tsunami and earthquake in March 2011 which wrecked the Fukushima power plant.

Previously, about 30% of Japan's power was nuclear-generated.

Prime Minister Shinzo Abe has lobbied for a restart, arguing that the shutdown has hurt the economy by forcing Japan to import expensive fossil fuels to make up the power shortfall.

But public anxiety about the safety of nuclear power remains high.

None of the plants has yet re-started. However, last week a Japanese court rejected an attempt by local residents to halt the restart of two nuclear reactors at the Sendai plant in Kagoshima prefecture.