The Mexican government says teacher testing will go on as planned, despite the objections of radical teachers' unions.
The teachers had vowed to disrupt the June 7 midterm elections unless constitutionally mandated teacher evaluations were dropped.
Many teachers in Mexico's under-performing schools have essentially inherited their posts.
On May 29, the government appeared to bow to their demands by announcing that the tests would be "indefinitely suspended." That was seen as a trade-off to keep elections peaceful. Only about 600 polling places suffered disruptions of a total of 148,836.
On Tuesday, the Education Department said testing will be held between September and November for about 360,000 public school teachers and almost 193,000 people applying for teaching positions. Teachers who refuse to take the tests will lose important bonuses.
The protesting teachers gathered at the Estela de Luz Monument to march towards the Revolution Monument. A second group coming from the Chamber of Deputies will join them.
Also, the National Coordination of Education Workers (CNTE) announced an indefinite national strike until the education reform is abrogated, the evaluation is suspended and the 25 teachers missing since last Sunday in Oaxaca are accounted for.