Opposition lawmakers crowd around Masahisa Sato (Center L), deputation chairman of the upper house special committee on security, at an upper house special committee session on security-related legislation at the parliament in Tokyo, Japan, September 17, 2015. [Photo/Agencies]

TOKYO - Japanese opposition lawmakers were battling on Thursday to prevent a vote on security bills that could allow troops to fight abroad for the first time since World War II, part of Prime Minister Shinzo Abe's agenda to ease the limits of the pacifist constitution.

Opponents argue it violates the constitution and fear it could ensnare Japan in US-led conflicts.

Objections by shouting and jostling opposition members, who have vowed to block passage before parliament disperses on Sept 27, delayed the start of an upper house panel whose approval is needed before the full chamber can vote the bills into law.

Thousands of demonstrators had rallied the day before near parliament chanting "Scrap the war bills" and "Abe resign" and were gathering in the rain again on Thursday.

Media said a ruling party official has threatened to send the legislation back to the lower house, which has approved the bills once and can vote them into law with a two-thirds majority in a second vote if the upper house fails to act.

The legal revisions include an end to a decades-old ban on defending a friendly nation under attack, or collective self-defence, when Japan faces a "threat to its survival".