MEXICO CITY Mexican authorities canceled in-person classes for more than 100,000 students on Thursday in multiple towns clustered around one of the country's most active volcanoes as its fiery activity intensified in recent days, raising health concerns.

A series of relatively small but powerful eruptions from the snowcapped Popocatepetl volcano prompted Puebla state officials to suspend both public and private school classes at all levels across 22 towns due to falling ash, according to officials.

Classes in the affected towns will temporarily be conducted online.

Falling ash, plus gases spewed by the eruptions, can be harmful when inhaled, and in a worse case scenario could trigger the need for massive evacuations.

Popocatepetl, meaning "Smoking Hill" in the native Aztec language Nahuatl, is one of the world's most closely monitored volcanoes. It rises only about 45 miles (72 km) southwest of Mexico City, home to about 9 million people or around 22 million including the metropolitan area.

Images shared on social media in the picturesque colonial-era main plaza of the Puebla state capital showed many people wearing masks to protect themselves from ash in the air.

Even though schools in the state capital remained open, officials recommended that locals avoid outdoor activities.

Mexico's disaster prevention center Cenapred maintained its alert level for the volcano at yellow on Thursday, which indicates moderate risk.

Cenapred reported that over the past 24 hours it has registered 154 exhalations of mostly ash and gas from Popocatepetl, one "minor" explosion and one volcano tectonic earthquake.

Popocatepetl is Mexico's second tallest volcano at 5,550 meters (18,209 ft) above sea level, and on clear days is a fixture of the Mexican capital's distant southern skyline.

(Reporting by David Alire Garcia; Editing by Richard Chang)