Niger issues warrant for opposition leader -source

* Arrest warrant issued for opposition leader

* Tandja rival to return to fight charge of money laundering

By Abdoulaye Massalatchi

NIAMEY, Oct 30 (Reuters) - Niger has issued an international arrest warrant for Mahamadou Issoufou, the main opposition figure leading the challenge against the West African nation's president, a legal source said on Friday.

Issoufou, who heads the Nigerien Party for Democracy and Socialism (PNDS), is one of dozens of former parliamentarians caught up in an investigation that critics say is an attempt to muzzle opponents of President Mamadou Tandja's moves to extend his powers.

Investigations have found "numerous transactions" of large amounts of money "from dubious origins" through Issoufou's accounts, leading to the charges of money laundering, the senior legal source told Reuters, asking not to be named.

Neither Niger's judiciary nor the government have made a statement on the arrest warrant.

Authorities in the uranium exporting nation say the investigation is aimed at rooting out graft.