Mexican officials say the death toll from the massive 7.1-magnitude earthquake has reached 286, but that count is expected to rise as scores of people are still missing.
Rescue teams in central Mexico have been working around the clock among the flattened buildings since Tuesday in search of survivors.
President Enrique Pena Nieto has declared three days of national mourning.
He praised Mexicans' rapid response to the disaster, while stressing the priorities remain saving lives and getting medical attention to those in need.
"I need to recognize the volunteers who are unconditionally helping those who need it," Pena Nieto said.
"Once again, Mexicans have demonstrated that the strength of solidarity is much greater," the president's office posted in a tweet that included a video showing thousands of people involved in relief efforts.
In addition to the local response in Mexico City and the states of Morelos, Puebla, Mexico, Guerrero and Oaxaca, help is coming from other nations.
The United States Agency for International Development (USAID) sent a team of more than 60 disaster responders and tools and medical equipment to Mexico City Thursday.
"They'll be working closely with Mexican disaster authorities to help rescue earthquake survivors and assess structures for earthquake damage, said USAID Administrator Mark Green.
A 32-member search and rescue team from Panama arrived with two dogs Wednesday.
The Israeli military said it would send a team of 70 people to help with rescue efforts by providing engineering assistance.
Pena Nieto said his government has also accepted the technical and specialized help offered by Spain, Japan and other Latin American countries that have experience dealing with the type of disaster his country is facing.
Mexico City's international airport sustained some structural damage, but Pena Nieto said the airport has returned to normal operations.
Canadian travel blogger Mike Corey was in Mexico City when the earthquake hit. The scariest part of the ordeal, he said, was the loss of cell reception, "which is not good if you're trapped underneath things."
Pope acknowledges victims
Pope Francis acknowledged the victims of the earthquake during an open prayer Wednesday in St. Peter's Square, saying he wanted to "express my closeness and prayer to the dear Mexican people."
The quake hit less than two weeks after another earthquake killed more than 90 people in the country's south. The U.S. Geological Survey said the two quakes appeared to be unrelated.
The earthquake struck exactly 32 years after an 8.0 temblor killed nearly 10,000 people in and around Mexico City.
Fern Robinson, Victor Beattie, Steve Miller, Celia Mendoza contributed to this report.