A leader of a Belarusian opposition party, Anatoly Lebedko, was being held in a Moscow airport on Tuesday for traveling with an invalid passport, a Belarusian law enforcement source said.Anatoly Lebedko, the chairman of the opposition United Civil Party of Belarus, showed border guards at Domodedovo Airport a Belarusian passport which he had earlier reported lost."It turned out that Lebedko did not in fact lose the passport and crossed into Russia with the document, which had already been invalidated," the source said.The valid passport, issued after Lebedko said he lost the old one, was seized while the opposition leader was detained for taking part in last year's December 19 rallies in Minsk.