(Recasts, adds details, protest by opponents of change)
By Minami Funakoshi
TOKYO, June 30 (Reuters) - Thousands of people marched in Tokyo on Monday to denounce a landmark shift in security policy by Japanese Prime Minister Shinzo Abe to ease constitutional constraints that have kept the military from fighting abroad since World War Two.
The protesters, including students, pensioners and women working at home, massed in front of Abe's office on the eve of a cabinet meeting expected to endorse what some analysts describe as the biggest shift since Japan set up armed forces in 1954.
An opinion poll published on Monday by the Nikkei business daily showed 50 percent of voters oppose dropping the ban compared to 34 percent who support the change.
Organisers said 10,000 demonstrators joined the march to the sound of drums, chants and saxophones. Some carried banners saying: "I don't want to see our children and soldiers die" and "Protect the constitution".
"If the prime minister changes the interpretation of the constitution every time, the constitution won't function," said Ayumi Yamashita, 51, her voice fading among chants from the crowd of "Don't let us go to war!".
Yuriko Umehara, 34, a construction company worker, said the change was a threat to peace. "To change an interpretation of the constitution, citizens should vote," she said.
Police put the number of participants at several thousand.
On Sunday, a man set himself on fire at a busy Tokyo intersection in an apparent protest against the change, police and witnesses said.
The cabinet is expected to adopt a resolution revising a longstanding interpretation of the constitution drafted by the United States after Japan's World War Two defeat. The junior coalition partner in Abe's government has indicated it will back the change.
Legal revisions needed to implement the change must still be approved by parliament, which could impose further restrictions in the process.
WIDENING MILITARY OPTIONS
The change in policy will significantly widen Japan's military options by ending the ban on exercising "collective self-defence" or aiding a friendly country under attack.
It will also relax limits on activities in U.N.-led peacekeeping operations and "grey zone" incidents short of full-scale war, according to a draft government proposal made available to reporters last week.
Since 1945, Japan's military has not engaged in combat. While successive governments have stretched the limits of the pacifist charter not only to allow the existence of a standing military, but also to permit non-combat missions abroad, its armed forces are still far more constrained legally than those of other countries.
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.
Critics say the change will gut Article 9 and make a mockery of formal amendment procedures.
"The constitution should check government powers, but Abe is using his powers to change it," protester Umehara said. (Additional reporting by Linda Sieg; Writing by Antoni Slodkowski; Editing by Ron Popeski)