A man has set himself on fire at a busy intersection in Tokyo in an apparent protest against Prime Minister Shinzo Abe's plan to ease limits of the country's pacifist consitution, police and witnesses said.

The incident at a busy intersection on Saturday came as Japan was poised to make a historic shift in its defence policy by ending a ban that has kept the military from fighting abroad since World War Two.

Abe's cabinet is expected to adopt as early as Tuesday a resolution revising a long-standing interpretation of the US-drafted constitution to lift the ban after his ruling party finalises an agreement with its junior partner.

It was not immediately clear whether the man survived.

A police spokeswoman confirmed the incident, which took place near bustling Shinjuku station, but would not provide further details.

Witnesses said the man, seated on pedestrian bridge, used a megaphone to protest plans to end a ban on exercising "collective self-defence", or helping a friendly country under attack.

The planned change in defence strategy marks a major step away from post-war pacifism and widens Japan's military options.

Conservatives say the charter's war-renouncing Article 9 has excessively restricted Japan's ability to defend itself and that a changing regional power balance including a rising China means Japan's security policies must be more flexible.

The change will likely rile an increasingly assertive China, whose ties with Japan have chilled due to a maritime row.

Mutual mistrust and the legacy of Japan's past military aggression have soured ties with its neighbour, but will be welcomed by Tokyo's ally Washington, which has long urged Japan to become a more equal partner in the alliance.

Source: Agencies