MOSCOW, SEPT. 28 -- President Mikhail Gorbachev, struggling to maintain central control over Soviet political and economic life, plans to transform 550 armament factories to civilian production, his spokesman said today.

Gorbachev and top aides met to complete the conversion plan as representatives of seven Soviet republics agreed to bypass central authorities in working out a new system of economic and political cooperation.

The national legislature, or Supreme Soviet, has been unable to pass a cohesive reform plan, and on Monday granted Gorbachev extraordinary powers to transform the national economy from a planned to a market system.

Representatives of nine of the 15 Soviet republics and Mayor Anatoly Sobchak of Leningrad, dissatisfied with Gorbachev's reforms, met in the Estonian capital, Tallinn. Officials from Russia, the largest and wealthiest republic, as well as the three Baltic republics, Armenia, Kirghizia and Moldavia signed a protocol establishing an Economic Consultative Committee to coordinate their own shift to a market economy. Representatives from Tajikistan and Byelorussia attended the session but did not sign the protocol, the Estonian news agency said.

Another group of reformers, led by Moscow Mayor Gavril Popov, held a news conference to express concern about the powers granted to Gorbachev. Their statement complained about "the dangerous situation created by the capitulation of the president to conservative forces, the worsening of relations between the center and the republics, especially Russia."

Gorbachev's plan to shift hundreds of defense plants to civilian production appeared designed to quell fears of a military resurgence and to assure people that the central government is acting to meet consumer demands.

Presidential spokesman Vitaly Ignatenko told reporters that Gorbachev was meeting tonight with members of his Presidential Council to complete plans to shift the 550 arms plants over the next five years to the production of processed food, medicine, electronics and other goods desperately needed in the faltering Soviet economy.