The Popocatepetl volcano outside of Mexico City has been spewing glowing rock fragments as far as 1km (0.6 miles) down its slopes.
Columns of ash have been shooting from more than 60 openings in the crust of the 5,450m (17,900-foot) volcano.
Residents of surrounding villages said they were awoken by a low-pitched roar coming from the mountain.
Popocatepetl's last major eruption was in 2000, when thousands of people had to flee their homes.
Local resident Aaron Sanchez Ocelotl said the eruptions sounded "like the roaring of the sea".
"Up on the mountain, it feels incredible," he said.
Gregorio Fuentes Casquera, the assistant mayor of the town of Xalitzintla, 12km (7 miles) from the summit, warned people to take the volcano's renewed activity seriously.
"This buzzing, this roaring isn't normal," he said, asking residents to be ready to evacuate.
On Tuesday, Mexico's National Disaster Prevention Centre (Cenapred) raised the alert around the volcano to "yellow stage three", the third-highest level.
Cenapred officials said the alert could remain for weeks or months until the volcano's activity diminished.
Popocatepetl, which means "Smoking Mountain" in the indigenous Nahuatl language, is Mexico's second-highest peak.