MEXICO CITY (Reuters) - The leader of Mexico's biggest opposition party is stepping aside temporarily to seek a congressional seat next year at which time he could resume his post as party leader.

Gustavo Madero, head of the center-right National Action Party (PAN), was immediately replaced at the helm by his deputy, the party said in a post on Twitter on Tuesday.

Madero has been grappling with an internal power struggle over how far his party should have cooperated with President Enrique Pena Nieto's centrist Institutional Revolutionary Party on a wide-ranging economic reform agenda.

The PAN's new interim president is 35-year-old Ricardo Anaya, previously the party's second-ranking official as well as a former federal congressman who served as president of Mexico's lower chamber of Congress earlier this year.

Shortly after the party's announcement, Madero told reporters he would seek a congressional seat representing his native Chihuahua state in Mexico's mid-term elections next year.

In response to a reporter's question, Madero noted that the PAN's internal rules would not prohibit him from serving again as party president next year regardless of how he fares in his congressional race.

He is also widely expected to mount a bid for the presidency in 2018.

The leaders of Mexico's political parties are powerful figures who directly appoint their party's leaders in Congress in addition to setting their faction's legislative agenda.

(Reporting by David Alire Garcia; Editing by Lisa Shumaker and Ken Wills)