VICHY--Bombs went off almost stmultaneously in the 10 largest cities of unoccupied France early today causing widespread destruction and revealing the existence of a nation-wide plot against Axis collaborationists.

The outbreak was timed one day ahead of a national convention of Jacques Doriot's pro-Nazi PPF party in Paris and his regional offices were tarets for some of the bombs.

There were two explosions in Vichy, one at 3 a.m. and one at 4 a.m. The first one demolished the Doriot headquarters here and the second wreeked the propaganda office of Marshal Henri Philippe Petain's government. It was the first time a government office has been attacked in the unoccupied zone. The explosions came at the stroke of the hour, and obviously were time bombs.