Lawmakers and activists on Thursday slammed the imminent dispatch of Maritime Self-Defense Force destroyers to the Gulf of Aden, calling it irrational and a threat to the Constitution.

"The dispatch could very well induce the first use of armed force overseas by the Self-Defense Forces," Upper House member Mizuho Fukushima, who heads the Social Democratic Party, said at a rally in Tokyo.

Two MSDF destroyers are expected to be given orders on March 13 to guard Japan-related vessels off Somali shores, under the maritime police action provision of the Self-Defense Forces Law. The government is also drafting a new permanent law that would enhance the MSDF's rules of engagement against Somali pirates.

"To allow the dispatch under the circumstances would become a precedent that would allow the dispatch of the SDF anywhere, anytime, at any cost," Fukushima said, describing the act as unconstitutional.

The 4,650-ton Sazanami and 4,550-ton Samidare will each carry two patrol helicopters and 200 crew members from the MSDF base in Kure, Hiroshima Prefecture.

While weapons use is permitted only for self-defense under the police action provision, the new antipiracy law, which a policy panel of the Liberal Democratic Party-New Komeito ruling bloc approved Tuesday, proposes that the MSDF be allowed to use arms in a precautionary manner against approaching pirate boats.