An Afghan bus passenger who walked most of the length of the Salang Pass tunnel immediately after the reported Nov. 2 explosion has put the number of Soviet soldiers and Afghan civilians killed in the disaster at 350, Western diplomatic sources here said today.

The passenger's account, which the diplomatic sources conceded had been passed on to them by a third unidentified person in Afghanistan and not by the passenger, was portrayed here as the most comprehensive report of the secrecy-shrouded disaster yet. It contrasted sharply with initial reports from Western sources in Kabul, which said that as many as 700 Soviet soldiers and at least 400 Afghan civilians suffocated in the 1.7-mile-long tunnel 70 miles north of the capital.

In the eyewitness' account quoted by the diplomats, a southbound gasoline tank truck collided head-on with the lead vehicle of a column of armored personnel carriers; the resulting fire, coupled with exhaust fumes, asphyxiated motorists sitting in their vehicles to keep warm.

This account contradicted reports that Soviet soldiers, thinking a guerrilla attack was taking place, prevented terror-stricken passengers from leaving, the sources said.

The passenger was quoted as saying he walked out of the north end of the tunnel and that when he returned the next day to retrieve his belongings, he saw military and civilian drivers slumped over the wheels of their vehicles.