MAPUTO — Mozambique opposition leader Venancio Mondlane returned from self-exile on Thursday, defiantly claiming to have won a hotly contested October election, but security forces later used tear gas to disperse supporters who had gathered to greet him. The disputed Oct. 9 poll, which Mondlane says was rigged, has sparked demonstrations over the past few months in which hundreds of protesters have been killed in the southern African nation of 35 million. “My return does not result from any political agreement. My return is a unilateral decision to be in Mozambique,” he told reporters at Maputo airport upon his return.