* Ban, Clinton urge Haiti to make use of U.N. support
* Haiti's "window of opportunity" is limited, Ban says
* Demonstrators protest against party exclusion from polls
(Recasts, adds quotes, details)
By Patrick Worsnip
PORT-AU-PRINCE, March 9 (Reuters) - U.N. Secretary-General Ban Ki-moon and former U.S. President Bill Clinton visited Haiti on Monday and urged the Caribbean state to use international backing to haul itself out of grinding poverty.
Ban and Clinton are on a mission to promote an anti-poverty action plan for the poorest nation in the Western Hemisphere.
Speaking to students at an educational center, the U.N. chief said Haiti had a "window of opportunity" because of the presence of a U.N. peacekeeping mission on its soil and because of the country's tariff-free access to the U.S. market.
"This window of opportunity is not unlimited. It is very limited. You must seize this opportunity," said Ban, who was due to meet later with President Rene Preval.
"That is why President Clinton and I are here personally to first of all demonstrate our solidarity and send a very strong message to the international community that we need Haiti to be able to emerge as a very stable, democratic and prosperous country in the region," he added.
The United Nations has some 9,000 peacekeepers in Haiti, which has long been afflicted by political instability and violence and was heavily damaged by hurricanes last year.
U.N. officials say the anti-poverty plan focuses on job creation, food security, reforestation of the almost treeless country and provision of basic services such as healthcare.
Clinton told the students that 200 years ago Haiti had been the richest part of the region because of its natural resources. "You can be again because of the resources in your mind and your heart," he said.
Clinton and Ban were visiting just over a month before Senate elections in Haiti, already controversial after the nation's biggest party, The Lavalas Family Party associated with exiled former President Jean-Bertrand Aristide, was barred on a technicality.
POSSIBLE RECOVERY SEEN
As they visited, several thousand supporters of the Lavalas Family Party staged a demonstration to protest against its barring from the Senate polls and to demand that U.S. President Barack Obama arrange Aristide's return to Haiti.
They waved banners which read "Ask Obama to return Aristide" and "There can be no election without Lavalas," but there were no serious incidents.
Aides to Ban said he had been encouraged to visit Haiti by a report he had received from Paul Collier, an academic at Britain's Oxford University.
The report said that despite its problems Haiti was well placed to recover because of the U.S. trade concessions it enjoyed, its proximity to the U.S. market and its low labor costs. It recommended focusing on developing the garment industry and in growing mangoes as an export crop and to help reforest the nation, which suffers flash-floods and erosion.
Accompanying Clinton was Wyclef Jean, a Haitian-born hip-hop star who heads a charitable foundation that seeks to help the impoverished state.
A U.N. statement said Ban had asked Clinton to accompany him because of his attention to Haiti during his 1993-2001 presidency and a "call to action" on Haiti at a session of his Clinton Global Initiative foundation last September.
The hurricanes that hit Haiti in August and September were estimated to have killed some 800 people and caused $1 billion worth of damage.
The United Nations launched an appeal last September for $108 million for hurricane relief, but less than half of that amount had been pledged as of December. A high-level donor conference is scheduled for next month in Washington.
Immediately after Ban's visit, the U.N. Security Council will stage a three-day trip of its own to assess progress in Haiti and how the U.N. mission is fulfilling its mandate. (Additional reporting by Joseph Guyler Delva; Editing by Pascal Fletcher and Eric Walsh)