Thousands of anti-nuclear demonstrators returned to Japanese Prime Minister Yoshihiko Noda's executive mansion in central Tokyo Friday for a weekly rally.

The gathering, organised on Facebook and Twitter, has been held every Friday since late March and gathered momentum after Noda approved the restart of two nuclear reactors in western Japan in June.

An AFP reporter estimated the crowd to be in the thousands, nearly 18 months after a massive earthquake and tsunami killed 19,000 and sparked reactor meltdowns at the Fukushima Daiichi nuclear power plant.

"We will continue calling for this rally at least this month," Taichi Hirano, 27, one of the organisers, told AFP outside the premier's mansion.

Japan turned off its stable 50 reactors in the wake of the disaster on March 11, 2011, but restarted two of them citing possible summertime power shortages.

Radiation from the plant, some 220 kilometres (140 miles) northeast of Tokyo, has been spread over a large area in the worst nuclear accident since the 1986 Chernobyl disaster.