A man set himself on fire in central Tokyo in protest at a proposed law which could allow Japan to deploy its military overseas.
The man was taken to hospital after being hosed down but his condition was not immediately known, officials said.
Japan's government could make the change to its pacifist constitution as early as next Tuesday.
The US-drafted constitution bans war and "the threat or use of force" to settle international disputes.
Witnesses said the middle-aged man, wearing a suit and tie, climbed onto a pedestrian bridge at Tokyo's Shinjuku station.
"He was sitting cross-legged and was just talking, so I thought it would end without incident," one eyewitness told Reuters. "Then all of a sudden his body was enveloped in fire."
Reports said the man used a megaphone to shout for over an hour about the change to Japan's constitution.
Video shown on national television in Japan showed the flames being extinguished by officers.
Japan has well-equipped and well-trained armed forces but there are severe restrictions on them being deployed abroad.
Under Article 9 of its post-war pacifist constitution, Japan is blocked from the use of force to resolve conflicts except in the case of self-defence.
But Japan's Prime Minister Shinzo Abe says he wants a new interpretation of the constitution to be agreed on.
The move has split opinion inside the country. Critics of the move warn against what they see as increasing militarism, while conservatives argue that the restriction is a double standard forced upon Japan.
Mr Abe's plan has led to criticism from China, whose relations with the Japan have become strained over territorial disputes in East China Sea.
Correspondents say the move will likely please the US, with whom Japan has a long-standing security treaty.