MOSCOW — Fighting flared yesterday between opposition forces and supporters of President Zviad Gamsakhurdia in Tbilisi, the Georgian capital."Illegal armed groups led by [opposition leader] Tengiz Kitovani attacked government headquarters from two sides and opened fire," a government statement said. Journalists said heavy artillery was being used against the building and no return fire was apparent. Mr. Gamsakhurdia, accused by the opposition of dictatorial rule and of muzzling the press, was believed to be inside, they said. The news agency Tass said the opposition was demanding the release of political prisoners, freedom of the media, and permission to demonstrate freely outside government headquarters. The Georgian Iprinda news agency quoted Georgian radio as saying appeals were being broadcast for people to gather around the government headquarters to protect it.