The march began in El Puerto de Santa Maria and ended in Fuentebravia, near the gate that leads to base housing.

Thousands more marched to military bases in Morón, Zaragoza, Albacete and Torrejon. Base officials in Rota told Americans to avoid the demonstrations.

“The Rota base is the U.S. aircraft carrier in Spain for the war in Iraq,” said Gaspar Llamazares, the head of the United Left coalition leading the march to Rota.

Under bilateral defense accords, U.S. forces can use the Rota base and the Morón Air Base, near Seville, for refueling. About 3,000 U.S. active-duty personnel are stationed at the two bases.

The Rota base also serves as a field hospital. Twenty-four U.S. soldiers wounded in the Iraqi war were flown there over the past week. The base currently has 116 beds but is being expanded to take in up to 500 patients.

The protesters also lashed out at the Spanish government’s policy of allowing U.S.-led coalition forces to use its airspace.

Local authorities said the marches, organized by leftist and anti-war groups, were peaceful, with no injuries or arrests.

In Barcelona, demonstrators attended a massive peace concert.

The government of Prime Minister Jose Maria Aznar is one of the staunchest supporters of the United States in the Iraqi conflict. However, polls show Spaniards are overwhelmingly opposed to the war.

A survey published Sunday in Madrid’s leading newspaper, El Pais, showed Aznar’s party slipping six points behind the opposition Socialist party as a result of his support of the Bush administration.

Aznar’s center-right Popular Party would take 36 percent of the votes if elections were held now, compared with 42 percent for the Socialists, said the Institute Opina poll conducted on last Tuesday and Wednesday.