Thousands of striking teachers seized two of Mexico City's central thoroughfares on a march to the president's residence on Wednesday after losing their battle to block new educational reforms less than 24 hours earlier.

The teachers disrupted the centre of the city for at least the 14th time in two months, decrying a plan designed

to break union control of Mexico's education system and, they say, damage education in Mexico's poor south in the process.

President Enrique Pena Nieto had dashed their hopes of blocking the overhaul when he signed a new testing system into law on Tuesday.

But the protests have continued in many Mexican cities.

Teachers scuffled with riot police on Wednesday at a police line set up to keep protesters from blocking a major road.

City officials reported 15 police were injured as protesters seized plastic riot shields from officers.

The teachers say their focus has shifted from blocking the reform to protecting their existing rights and privileges, which they claim are substantially reduced under the new measures.

Constituyentes Avenue, one of Mexico City's main thoroughfares, was empty of its usual heavy traffic as thousands of teachers gathered on both sides of the six-lane road to prepare to march to the gates of the park surrounding the presidential residence Los Pinos.

Teachers also blocked roads and city centres in at least 15 other Mexican states in response to calls for a national day of protest led by the National Education Workers Co-ordinating Committee CNTE, the smaller of the country's two main teaching unions.

The teachers claim their tactics aren't meant to win the sympathy of Mexico City residents because the media is controlled by the Mexico elite, rendering appeals to popular opinion pointless.

The powerful will only listen to power, they say, and the teachers' main power is their ability to shut schools and make life inconvenient in Mexico's economic and political centres.