Dozens of people have been killed and thousands forced onto the streets of Mexico City a powerful 7.1 magnitude earthquake.
The governor of Morelos said at least 42 people had been killed in his state, while at least five people were reported to have died in neighbouring Puebla.
Thousands were forced on to the streets of the capital by the power of the quake.
Panicked workers fled as clouds of dust rose up from the crumbling facades of damaged buildings after the tremor struck.
The centre of the quake struck more than 160km away from Mexico City in the state of Puebla.
The head of Mexico's emergency services has said all flights to the capital's airport have been suspended.
— USGSted (@USGSted) September 19, 2017
Mexico City's mayor said there were reports of people trapped in collapsed buildings and local TV footage showed rescuers frantically trying to dig into rubble with pickaxes.
Speaking minutes after the earthquake struck, resident Georgina Sanchez sobbed: "I'm so worried. I can't stop crying. It's the same nightmare as in 1985."
Center of Mexico City right now after 7.4 earthquake. Scary. Hope folks are ok. Video shot by a friend in DF pic.twitter.com/tlYtpEShcB
— David Prager (@dlprager) September 19, 2017
Mexican President Enrique Peña Nieto was in the air travelling to the city of Oaxaca when the tremor struck.
Writing on Twitter, he pledged to immediately return to the capital to address the situation.
He has called the country’s National Emergency Committee together to "assess the situation and to coordinate actions."
He convocado al Comité Nacional de Emergencias para evaluar la situación y coordinar acciones. El PlanMX ha sido activado.
— Enrique Peña Nieto (@EPN) September 19, 2017
People evacuated from office buildings gather in Reforma Avenue after an earthquake in Mexico City, 19-09-2017. Image: AP Photo/Rebecca Blackwell.
A civil protection official in Puebla said two people had been killed after a school collapsed, while Mexico State's governor said a quarry worker had been killed by a rockslide and another victim was hit by a falling lamppost.
Officials asked people not to smoke in the streets of the sprawling city - which has a population of 20 million - warning of possible ruptured gas pipes.
Electricity and phone lines were down in parts of the capital.
"We got out really fast, leaving everything as it was and just left," said Rosaura Suarez, as she stood with a crowd on the street.
Alfredo Aguilar, 43, said the quake was "really strong - buildings started to move."
He added that he saw a woman fainting as "people started to run."
— Benjamin Alvarez (@BenjAlvarez1) September 19, 2017
— Justin Hendrix (@justinhendrix) September 19, 2017
It comes after a powerful 8.1 magnitude earthquake killed at least 90 people in the country earlier this month.
1985 Mexico City Earthquake
This evening’s tremor struck on the 32nd anniversary of a 1985 quake that caused major damage in the capital.
The 1985 event resulted in the deaths of at least 5,000 people.
This afternoon, just two hours and 15 minutes before the tremor, buildings across the city held practice evacuations drills to mark the anniversary.
It remains unclear exactly what damage has been caused by today's quake - or whether there have been any casualties.
Patients lie on their hospital beds after being evacuated following an earthquake in Mexico City, Tuesday, 19-09-2017. Image: AP Photo/Marco Ugarte
US President Donald Trump tweeted: "God bless the people of Mexico City. We are with you and will be there for you."