Mexican police have arrested a teachers’ union leader accused of siphoning off money raised illegally from union members in the southern city of Oaxaca.
Ruben Nunez has led a decade-long fight for better pay and working conditions.
His supporters say Mr Nunez is innocent, accusing the Mexican government of having political reasons to arrest him.
They have set up roadblocks around Oaxaca to demand his release.
The union has also called for demonstrations in Mexico City on Tuesday and Friday.
Prosecutors have accused Mr Nunez of money laundering.
“He raised more than 24m pesos ($1.3m; £900,000) illegally between 2013 and 2015,” said Prosecutor Gilberto Higuera.
For two years the union demanded a 3.5% cut of purchases made by unionised members from some businesses in Oaxaca, added Mr Higuera.
The money was taken from the education workers’ payslips, which prosecutors say was controlled by the union.
“We have not ruled out the possibility of involvement from other people [from the unions in Oaxaca] in other illegal activities,” said Mr Higuera.
Mr Nunez was arrested in the early hours of Sunday and taken to the federal prison of Hermosillo, in the north-western state of Sonora.
Fellow union activist Francisco Villalobos was arrested in Oaxaca.
Mr Nunez is the leader in Oaxaca of the National Co-ordination of Education Workers (CNTE), founded in 1979 as a dissidence of the mainstream SNTE.
The CNTE described the two activists as “political prisoners” and “hostages of the federal government and of [Oaxaca] Governor Gabino Cue”.
The union opposes education reforms introduced in 2013 by President Enrique Pena Nieto, which include new measures to evaluate teachers.