Colombia on Wednesday declared three days of mourning for victims of an earthquake that killed more than 200 people and demolished scores of buildings in the west of the country.

At 7.4 magnitude, Monday's quake was the strongest to hit Colombia this century.

The government said flags would be lowered to half-staff on official buildings, as well as at Colombian embassies abroad, "in honor of all the dead," who are now numbered at 216.

In the cities of Pereira and Cali, emergency crews worked through the night with cranes, dogs and heavy machinery in the hunt for survivors, while volunteers formed human chains to clear debris by hand.

There was joy when rescuers pulled out 32-year-old Daniela Largo alive after 35 hours trapped in the rubble of a Pereira hotel.

"We were losing hope and then we received a call," her mother Sandra Milena Perez told AFP.

Perez said a local resident heard cries for help beneath the rubble and joined neighbors in removing debris before professional rescuers arrived.

"I thank him with all my heart because thanks to him today we are happy... a son is waiting for her at home," she said.

The first two days after an earthquake are crucial for finding survivors under rubble, with some experts saying that after a 72-hour window, hopes quickly fade.

From the air, neighborhoods in Cali appeared flattened, with piles of grey rubble replacing entire blocks of homes and apartment buildings.

Nearby, cars lay crushed beneath fallen walls and concrete slabs.

The quake overwhelmed parts of the health system, turning the rescue effort into one of Colombia's largest disaster responses in years.

Many survivors spent the night outdoors, either because their homes were damaged or because they feared aftershocks. 

One park in Pereira was transformed into a makeshift shelter with tents and mattresses.

"We lost everything, but we're alive," said 23-year-old John Tabasco, who was camped there with his wife and seven-month-old son.

In Cali, part of the Hospital Universitario del Valle collapsed during the quake, forcing doctors to move patients into a parking lot and treat some in the open air.

Many Colombians mobilized to help.

Thousands brought water, food and supplies to affected neighborhoods, while long lines formed outside blood donation centers.

"It's about wanting to help your community, your city and your country," said blood donor Ferney Cabrera.

President Abelardo de la Espriella, who was just sworn in on Friday, declared a state of emergency and promised rent subsidies for affected families Tuesday during a visit to Pereira.

The United States announced $15.5 million in emergency aid, while messages of support and offers of assistance arrived from around the world.