Costa Rica ? Nobel Peace Prize winner Oscar Arias returned to the Costa Rican presidency on Monday, hoping to use his skills as a mediator to unite a country sharply divided over free trade with the United States.

The 65-year-old Arias, best known for winning the Nobel Peace Prize in 1987 after helping broker an end to Central American civil wars, was sworn into office promising he will work to stabilize Costa Rica’s economy through increased investment in education, job creation and a government that is committed to ethics and openness.

But as Latin American leaders and first lady Laura Bush gathered for Monday’s inauguration at Costa Rica’s National Stadium, about 4,000 union members, students and academics marched outside the forum to demand that Costa Rica not ratify the free trade agreement known as CAFTA. Costa Rican officials have signed the pact, but it has not been ratified in the country’s Congress.

Arias’ strong support of the trade pact may have nearly cost him his bid to return to the presidency, which he held from 1986 to 1990. He argued in his campaign that the trade deal will help revitalize the country’s stagnant economy.

Nicaragua, Guatemala, Honduras, El Salvador and the Dominican Republic already have joined.