MANAGUA, Nicaragua (Reuters) - Sandinista revolutionary leader Daniel Ortega conceded defeat in Nicaragua's presidential election on Monday and accused his old enemy, the United States, of a ``dirty campaign'' to prevent his return to power.

Ortega fell to a decisive defeat and blamed U.S. criticism of him ahead of Sunday's election for scaring voters, upsetting his chances of winning back the presidency 11 years after he was voted out of office.

``There was very strong interference from external forces; that undoubtedly tends to polarize the elections,'' Ortega said after results showed him trailing way behind his conservative rival, Enrique Bolanos, of the ruling Liberal Party.

``The fear element was used again; there was a return to a dirty campaign; a terror campaign was used,'' Ortega said.

Ortega was a senior guerrilla commander in the Sandinista revolution in 1979 and then led Nicaragua for 11 years, fighting a bitter military campaign against U.S.-backed Contra rebels at the height of the Cold War.

He was ousted in a 1990 election and lost again six years later, both times at the hands of U.S.-backed candidates.

Ortega projected a softer image this time around, bringing former critics into his alliance and abandoning the socialist rhetoric of the 1980s to preach peace, reconciliation and free market economics with a human face.

He wore pink shirts instead of the combat fatigues of his past and ran on the slogan ``Love is stronger than hate.''

But political opponents at home and in Washington poured scorn on the ``new look'' Ortega and his transformation failed to win over enough support to take victory.