"When we get an idea of the toll it will be measured in the hundreds," said the doctor, who was covered in blood and nursing an injured left arm.
Numerous buildings had been flattened in the 7.0 quake, which was followed by three strong aftershocks.
Police and UN vehicles could be seen transporting the injured for medical attention.
In Washington, the State Department said it feared a "serious loss of life."
Spokesman PJ Crowley said US embassy staff reported scenes of destruction in and around the capital.
"They reported a lot of walls down. They did see a number of bodies in the street and on the sidewalk that had been hit by debris," he said.
Meanwhile, the United States, France, Canada and governments across Latin America were gearing up Tuesday to help the stricken country.
US President Barack Obama said his government stood "ready to assist the people of Haiti," as the State Department, USAID and United States Southern Command mobilised, the White House said, "to coordinate an assessment and any such assistance."
In Paris, Foreign Minister Bernard Kouchner said France "expresses its complete solidarity" with Haiti, adding that his ministry's crisis center had begun working "to mobilise and dispatch without delay urgent aid to Port-au-Prince."
Canada, which hosts an 80,000-strong Haitian community, said it was "deeply concerned" with events unfolding in Haiti.
Canadian Foreign Affairs Minister Lawrence Cannon said Canadian officials "are making contact with trusted humanitarian partners with a presence in the region to identify humanitarian needs resulting from this earthquake."
Brazil's President Luiz Inacio Lula da Silva said he was "very concerned" with the situation in Haiti, including the fate of some 1,200 Brazilian members of the 7,000-strong UN Stabilization Mission in Haiti, known as Minustah, which Brazil leads.
A Minustah employee in Haiti said that the force's headquarters had been largely destroyed in the quake.
In Bogota, the president's office said Colombia was "under alert and ready to respond to the Haitian authorities' call for help," adding that the defense ministry and emergency management services were coordinating upcoming assistance efforts.
Venezuela said it would send a 50-member "humanitarian assistance team" to Haiti in the next few hours. Foreign Minister Nicolas Maduro said the team would leave for Port-au-Prince early Wednesday bearing food and medical supplies for stricken Haitians.
Maduro also said he has been unable to contact Venezuela's ambassador to Haiti or Haitian authorities since the earthquake struck.
In Panama, the country's Vice President Juan Carlos Varela pledged that his government would provide assistance to the United Nations's Office for the Coordination of Humanitarian Affairs.
Varela pledged to provide "all the infrastructure required for the logistical coordination of cooperation that should be sent to this brother country."
Peru said its 205 peacekeeping troops in Minustah would help in rescue efforts in Haiti, adding that none of the soldiers was injured in the quake.
"The military personnel has nothing to report and will lend support to the quake victims to the best of their abilities," Peru's Joint Chiefs of Staff said in a statement.
Mexico's foreign ministry said it was keeping close watch on Haiti and had "activated a protection plan for Mexicans facing an emergency" in Haiti.
In the Dominican Republic, which shares the island of Hispaniola with Haiti, President Leonel Fernández called on the international community to help Haiti overcome a "real tragedy." He said his government was at the ready to send assistance.
A government spokesman said a Dominican Air Force plane was ready to pick up quake victims and that it was sending a team of sniffer dogs to search for victims in the rubble.
Meanwhile, in nearby Cuba -- only 80 kilometres from Haiti -- people in the eastern coastal town of Baracoa were evacuated to higher ground as a precaution, after the earthquake triggered a tsunami alert, the civil defence agency said.
The tsunami alert was lifted later, but Cuba said it "remains vigilant for possible aftershocks and rising sea levels in the eastern provinces," an agency statement said.
The initial quake was felt in Cuba's eastern province of Santiago de Cuba.