BUDAPEST, Hungary, Wed., Oct. 31--Heavy explosions and small arms fire rocked central Budapest early today as the deadline neared for Soviet troops to get out or be bombed by the Hungarian air force.

The Hungarian air force threatened to attack Russian troops and tanks involved in the battle of Budapest unless they leave the capital within 12 hours. The ultimatum, issued at 3 p.m. Budapest time, accused the Russians of "constantly violating the cease-fire."

Premier Imre Nagy, bidding to end a week of bloody civil war, announced the Communist one-party system will be abolished and called on the rebels to join in preparing for free elections taking in long-suppressed Hungarian political parties active before the Reds seized control in 1948.

Russia Contemplates Evacuation

MOSCOW, Oct. 30--The Soviet government announced tonight it is ready to discuss the disposition of Soviet troops in Hungary, Poland, and Romania with other Warsaw pact members.

An official announcement said that as a first step it has ordered its forces to evacuate Budapest "as soon as this is considered necessary by the Hungarian government."

The statement was accompanied by the uncharacteristic acknowledgement that "the further presence of Soviet armed units in Hungary may cause even further aggravation of the situation." It added that the Russians regret the bloodshed.