Disaster experts from the NGO are expected to arrive in the country today (14) to work alongside local specialists as they assess immediate needs and begin providing emergency food, water and shelter.
A massive appeal has been launched focused on helping children most vulnerable in the wake of the disaster. Plan, which works with 42,000 children in Haiti, has already released more than US $500,000 of emergency funds to help deal with the immediate aftermath.
Plan Haiti Director Rezene Tesfamariam said the devastation in the capital Port au Prince was on an "unimaginable" scale.
"People are trying to recover bodies from buildings all over the city. The emergency services are overworked and most of the bodies are being recovered by loved ones. They are using whatever they can, their bare hands or rudimentary tools like shovels or pick-axes," he said.
"Makeshift camps are springing up across the city and Plan is involved in assessing people's needs.
"I've seen villages destroyed by floods but this is something else. I've worked for Plan for 27 years and have seen all kinds of tragedies. I've seen refugees fighting for their lives and floods destroy communities but I've never seen anything like this."
Plan International's Director of Humanitarian Response & Disasters, Roger Yates said the Haitian people were doing their best to rescue people, but international help was vital.
He said: "In such a terrible time, outside support is going to be really important. Children are always the most vulnerable in these situations, particularly those who have lost parents and Plan staff are already working to help communities protect these children."
The NGO, which has been in the country since 1973, is working closely with government, emergency services and other organisations to respond to the immediate needs of children in the region and to assess the situation.
Having worked in Haiti for 36 years, Plan is well placed to know what assistance will be needed in the long term to endure communities back on their feet.
Initial aid will focus on providing clean water and sanitation, health care and counselling - education and setting up temporary schools will also be a priority as the recovery progresses.
Plan staff reported severe damage in Jacmel and Croix-des-Bouquets two of the areas where it operates. A tsunami alert was issued for the Haiti, the Dominican Republic and Cuba but was recalled shortly afterwards. Reports from Pedernales on the Dominican border with Haiti say that there was panic among people when the ocean pulled back, but then returned to normal.
Plan's UK office is appealing for funds to help protect children and provide the help and support for they will need.
"Our priority is assisting children and their families and getting people at risk into safe accommodation wherever possible. Also easing the psychological trauma upon people, which is not as obvious, will be very important as will ensuring child protection," says Plan UK's chief executive Marie Staunton.
Anyone wishing to make a donation or to find out more about Plan's work can visit www.plan-uk.org/haiti or freephone 0800 130 0230.