Japan's left-leaning labour confederations on Sunday called for an end to nuclear power generation, in May Day rallies held as emergency workers toiled to bring a quake-hit nuclear plant under control.

"Let us stop the government from promoting nuclear power generation and seek a change to its energy policy," Sakuji Daikoku, head of the National Confederation of Trade Unions (Zenroren), told a rally in Tokyo.

Japan Communist Party chairman Kazuo Shii also told the gathering at Yoyogi Park: "We will strongly demand the government make up its mind to withdraw from nuclear power generation and set a programme to reduce nuclear power plants to zero."

The rally was attended by some 21,000 people, according to the confederation which claims a membership of 1.2 million workers.

Another rally held in Tokyo on Sunday, sponsored by a smaller labour confederation named Zenrokyo, also raised the nuclear issue.

"Let us join our hands in switching Japan's society off from nuclear power," Social Democratic Party leader Mizuho Fukushima told the Zenrokyo rally at Hibiya Park outside the Imperial Palace.

The country's largest labour confederation, Rengo, which has 6.8 million members and supports the centre-left ruling party, the Democratic Party of Japan, held a subdued May Day rally in Tokyo on Friday, a public holiday.

The government of Prime Minister Naoto Kan had promoted the export of nuclear power plants until a 9.0-magnitude quake and monster waves ravaged the Fukushima Daiichi plant on March 11 and knocked out its cooling systems.