The Popocatépetl volcano, just 35 miles from Mexico City and only 20 miles from nearby city Peubla sent a mile high plume of ash into the air, putting thousands of people living within 10 miles of it on a yellow alert to be ready to evacuate should activity increase.
The Mexican capital is the world’s fourth most populated city and home to 20 million people, while Peubla has more than 6 million people living there, and all could be at risk in both cites in the event of a catastrophic eruption.
The volcano alert follows fears earlier this month that a second large volcano in Mexico - the 3,850 metres-high Colima in western Mexico - could be about to face a large scale eruption for the first time in 100 years.
Popocatépetl last saw a major eruption in 2000, but early warnings saw 41,000 people evacuated in advance, averting a major disaster.
In 2005 it had a minor eruption, but activity has been increasing since 2012, with this week's some of the most notable, prompting officials to issue the warning.
If the volcano experienced a massive eruption, an estimated nine million people would be in the blast zone and the impact on the atmosphere could be catastrophic.
Popocatépetl has erupted more than 20 times since 1519, according to records.
The glacier-covered peak is 5,426 metres-high.
Meanwhile Colima experienced a saw an ash cloud of 1.8 miles high expelled earlier this month.
This also put locals on standby for evacuations immediately afterwards.
A local report on wired.com said: "Two Mexican volcanoes have been increasingly active.
"Popocatépetl produced a number of explosions as lava reaches the surface at the active vent, as evidenced by incandescence spotted at the volcano’s summit.
"Most of the explosions produced ash-rich plumes that reached hundreds of meters over the volcano,
"Some officials are saying that there is a heightened danger of a large explosive eruption from Colima because it has been over 100 years since that last large eruption.
"The heightened explosive activity at Colima has prompted preparations for evacuations if the restlessness continues or increases."
The warnings come after two eruptions of significant volcanoes the same day on Tuesday.
It was before officials in Russia had to cancel flights near the Zhupanovsky volcano on Russia's eastern coast after it spewed lava FIVE MILES into the air, also on Tuesday.
Although the two volcanoes are separated by around 3,000 miles, they are on interconnected tectonic plates – sparking fears of a major seismic shift, more volcanic activity and even earthquakes and tsunamis, due to recent activity.