Trapped under tons of rubble and in complete darkness, Diana Pacheco's hopes were fading fast for making it out alive from a collapsed office building after a huge earthquake in Mexico City, despite rescuers' frantic attempts to reach her.

Then the trapped woman had a great stroke of luck: a series of short messages she had written and sent to her husband some 16 hours earlier lit up his phone screen.

"My love," the first message read. "The ceiling fell" "We're trapped" "I love you" "I love you a lot" "We're on the fourth floor" "Near the emergency stairway" "There's four of us."

Ms Pacheco's WhatsApp messages finally reached her husband Juan Jesus Garcia on Wednesday at 5:34am.

Garcia, 33, an Uber driver, had been waiting, often in tears, beside the collapsed building all night and immediately ran over to rescue workers.

Juan Jesus Garcia waited in tears beside the collapsed building all night for his wife Diana. ( Reuters: Anthony Esposito )

"It was like a miracle because I was the only one who got the message and since I was there with the rescue workers I talked to them and they could locate her," Mr Garcia said.

The messages on Mr Garcia's phone, seen by Reuters shortly after they were received could have been delayed due to erratic mobile phone coverage in parts of Mexico City after the quake, or the fact that Ms Pacheco's phone signal was blocked by the tons of concrete that kept her trapped in the collapsed building.

When asked whether WhatsApp messages can be delivered hours after they were sent in an area without good phone coverage, a spokeswoman for the company confirmed it was possible.

Ms Pacheco, a recruiter for a human resources and accounting firm, said she sent the messages shortly after the magnitude earthquake struck at 1:14pm on Tuesday.

"Those messages helped them know more or less where we were located," Ms Pacheco said from her hospital bed, her voice weak.

Using the information to pinpoint their location, rescuers freed her and the three other survivors.

People protest the "lack of results" from recovery efforts outside the premises of where Ms Pacheco was rescued. ( Reuters: Henry Romero )

Rescue operations were still underway on Friday at the building, where Ms Pacheco said there were 60 people on her floor alone at the time of the quake.

"I think there are people [alive] there because we had oxygen, air was coming in," she said.

Despite having bruises all over her body and wearing a neck brace, Ms Pacheco was generally in good health.

She said she tried to send WhatsApp and text messages to other people from under the building, as well as make phone calls and post on Facebook, but only the messages to her husband got through.

She said when the building fell, the force of two floors above collapsing violently knocked her down, but a wall of concrete stopped just short of crushing her and three of her co-workers.

Across Mexico City dozens of buildings tumbled into mounds of rubble. ( Reuters: Ginnette Riquelme )

They found themselves huddled together in a cramped space.

They screamed out every time they heard voices from outside the building.

"We heard them [rescue workers] when they asked us to yell or make noise, but regardless of how much we yelled they couldn't hear us," Ms Pacheco said.

Exhaustion, frustration sets in

Sorry, this video has expired Rescuers searched frantically for Maria Ortiz, a cleaner believed to be trapped under the rubble.

The quake, Mexico's deadliest in a generation, has already claimed close to 300 lives.

As the shock of this week began to subside, exhaustion crept in, along with growing discontent.

On Thursday, Mexico's Navy apologised for communicating incorrect information in a story about a girl supposedly trapped under a collapsed school in Mexico City.

A frantic effort had been made to reach the child, dubbed Frida Sofia by local media, but it turned out that the widely-publicised story had been false, leading to anger.

Francisco Ortiz questioned whether attention directed at trying to rescue the phantom girl had diverted resources from other places where they were desperately needed, like the apartment building where his sister, Maria, was trapped beneath debris.

Authorities had waited until Thursday (local time) to begin searching the building. The owner, Juan Salazar, said all the renters had been accounted for before realising that Ortiz's sister Maria, a maid, had been washing clothes on the roof when the quake struck.

Mr Salazar said he called civil protection and also implored passing brigades of rescue workers to help, but it was two days before rescue efforts began.

"It was negligence. Nobody wanted to take responsibility, neither the army nor Civil Protection," Mr Ortiz said. "We are hoping for something good from God."

On Friday afternoon after a full day's search, rescuers pulled Maria's body from the rubble.

A relative cries as Maria Ortiz's body is recovered. ( Reuters: Carlos Jasso )

Newlyweds spend honeymoon in makeshift shelter

In Mexico City, Jorge Daniel Huitzil and Erika Castillo Aparicio spent their first night as husband and wife sleeping beneath a tarp in a makeshift quake shelter.

They had plans to be married this week in a civil ceremony followed by a church service and a reception. But Tuesday's deadly magnitude-7.1 earthquake turned their world upside down.

Their apartment building was damaged so severely that it is too dangerous to live there, and the young couple wanted to postpone the nuptials.

But then they learned they would lose their slot at city hall — and the 1,019 pesos (about $US60) they had paid to reserve it.

"We were not able to cancel it, so we had our wedding today," a beaming Mr Huitzil said.

A family prepares to sleep on the footpath after their home was damaged. ( AP: Rebecca Blackwell )

Reuters