Water, ash and rocks are flowing down the flanks of the Popocatepetl volcano southeast of Mexico City.
Mexico's national disaster prevention agency says it has warned the communities near the volcano to be alert, but doesn't see any need for evacuations.
Authorities say similar activity in 2001 posed no risk to the population. Still, officials are urging people to stay at least seven miles from the crater.
The mountain has been shaking and spewing ash plumes since Monday evening.
The 5,450-metre volcano is about 40 miles southeast of the capital.