Mexican President Felipe Calderon, set to leave office in less than 48 hours, on Thursday signed a labor law overhaul he said will foster competitiveness and productivity while strengthening workers' rights.

In a ceremony at the Los Pinos presidential residence, the conservative Calderon said the measure introduces more than 300 changes to the 1970 Federal Labor Law.

The legislation introduces new hiring modalities, such as hourly pay, seasonal work and probationary periods for new hires; regulates outsourcing; and stiffens fines for companies who fail to meet their obligations.

It also includes measures to speed up labor dispute resolutions and open unions to greater public scrutiny, one of the legislation's most controversial aspects.

"From now on, labor leaders must be elected by free and secret ballot and no worker can be refused information about how their union dues are being spent," he said.

These changes were rejected by Mexico's most powerful unions, some of them closely tied to the Institutional Revolutionary Party, or PRI, which will return to power on Saturday with the inauguration of Enrique Peña Nieto as president.

Most of Mexico's largest unions were created during the 1929-2000 hegemony of the PRI and functioned as appendages of the then-ruling party.

The bill was submitted in September by the lame-duck Calderon and approved by Congress under a fast-track procedure.

Mexico's left says the new law strips workers of their rights, makes employment more precarious and undermines collective bargaining. EFE