MEXICO CITY - Protesters stormed Congress on Thursday for the second time in three days, breaking down a door before being pushed back by police and security guards.

The protesters, mostly teachers and farmers, caused less damage than Tuesday when many of the same demonstrators rode horses into the lobby of the building, hurled fire extinguishers at fleeing security guards and disrupted the legislative session for more than an hour.

Hundreds of federal police officers have since stood guard outside the legislature and the Attorney General's office has launched an investigation into the violence.

The second protest involved about 500 teachers who are demanding higher wages and say lawmakers have ignored their demands for salary increases. They have been camped out in front of the legislative building near downtown Mexico City for several days, living in tents and under tarps.

The teachers have been joined by farmers and ranchers angered by a provision of North American Free Trade Agreement will lift tariffs on several U.S. farm products in January. Some protesters have brought horses, cows and other farm animals with them to the encampment.

Lawmakers organized special commissions to hold discussions with the protesters Wednesday, but the talks failed to produce any major agreements.

"The truth is they don't want to talk. What they want to is throw Congress into chaos and impede end-of-the-year meetings," said one lawmaker, Guadalupe Castillo.