MEXICO CITY (Reuters) - Mexico's government said on Sunday it had arrested the leader of a dissident teachers' union for alleged corruption, redoubling efforts to impose its authority on an education reform that has sparked months of protests in the country.

Deputy attorney general Gilberto Higuera told a news conference that Ruben Nunez, head one of the most combative factions of the CNTE union, had been held on suspicion of receiving money of illicit origin through the organization.

"This is another step in the process of full application of the law, and respect for the rule of law," said Javier Trevino, deputy minister of education.

Nunez is secretary general of the CNTE's Section 22 in the southern state of Oaxaca, a longstanding hotbed of resistance to government efforts to reform the education system.

According to Higuera, Section 22 made a deal with companies supplying the teachers' union to charge an illegal 3.5 percent commission on sales, money its leaders then used for personal ends and to fund demonstrations by its members.

Between 2013 and 2015, more than 24 million pesos ($1.29 million) were acquired by Nunez as part of the scheme, he added.

The CNTE was defiant following Nunez's arrest.

"We're not going to give up our fight," Section 22 member Juan Garcia told a crowd of supporters in Oaxaca's capital, Oaxaca City.

Organizing marches and protests that have frequently caused chaos in the Mexican capital, the CNTE has spearheaded efforts to resist the education reform, in particular its mandate to carry out evaluations of teachers.

Critics argue many teachers in the poorer south lack the financial support to meet required standards.

President Enrique Pena Nieto has staked his name on the reform, describing it as the most important for Mexico's future.

In the past month the government has said over 4,000 teachers in the states of Michoacan, Guerrero, Oaxaca and Chiapas on the Pacific seaboard will be dismissed.

Two other senior figures in Nunez's group have also been arrested in recent days, and the three men are in prison in the northern city of Hermosillo, officials said.

($1 = 18.6235 Mexican pesos)

(Reporting by Dave Graham, Adriana Barrera and Jose de Jesus Cortes; Editing by Meredith Mazzilli)