LIMA, Jan 4 (Reuters) - Peru"s attorney general opened a preliminary investigation against leftist president Pedro Castillo on Monday on allegations of collusion and influence peddling in cases related to government public works contracts. The prosecutor announced the probe of Castillo for alleged influence peddling, focusing on meetings that Castillo had with representatives of companies vying for a contract to build a bridge in an Amazon area and another for the sale of fuel to the state-owned Petroperu, said a statement from the prosecution on Twitter. It said the president "improperly intervened" in the bidding process. Castillo, whose popularity has fallen rapidly after taking office at the end of July, has rejected the accusations. He says they are orchestrated by his opponents, who want to oust him. The administration"s press office was not immediately available to comment. Five of Peru"s former presidents have been investigated for corruption over recent years. Presidents of Peru are immune from prosecution during their five-year term, but they can be investigated.