MONTEVIDEO, Uruguay — José Mujica, a brash former guerrilla fighter, was elected president of Uruguay on Sunday, further cementing the hold of a leftist government credited with improving economic conditions in one of South America’s smallest countries.

With more than 90 percent of the vote counted Sunday night, Mr. Mujica, the candidate of the Broad Front coalition, was leading by about 10 percentage points in a runoff against Luis Lacalle, a former president running on the National Party ticket. Mr. Lacalle gave a concession speech on Sunday evening.

The victory of Mr. Mujica, 74, solidified the control the Broad Front has assumed over Uruguayan politics since the current president, Tabaré Vázquez, was elected in 2004. Mr. Vázquez pursued a pragmatic path of reforms with socialist and market-friendly elements that lowered unemployment and poverty while generating confidence among investors.

Uruguay’s Constitution does not allow for re-election, but Mr. Vázquez’s approval ratings in excess of 60 percent have strengthened the Broad Front, a polyglot movement that includes Communists and Christian Democrats.

“Tomorrow the commitment to our homeland continues,” Mr. Mujica declared in a speech here Sunday, with Mr. Vázquez by his side. “Thank you, Tabaré, for the continuation of this government.”

Tens of thousands of revelers, waving flags and setting off red flares, celebrated along the banks of the Río de la Plata despite a stiff wind and driving rain.

The Broad Front, a leftist movement, has followed an economic path closer to those pursued by Brazil, Chile and Peru than to that of Venezuela, Bolivia and Ecuador, which have chased off foreign investors by nationalizing industries.

Mr. Mujica campaigned on pledges to be a consensus builder who would continue the policies of Mr. Vázquez. The race pitted him against Mr. Lacalle, a neo-liberal who favors privatizing state firms and sought to do away with the income tax.

Mr. Lacalle, 68, was president from 1990 to 1995; his campaigns in 1999 and 2004 were derailed in part by allegations of governmental corruption while he was president.

The victory by Mr. Mujica, a farmer and Socialist senator, completed an improbable journey. He helped found the Tupamaro movement, inspired by the Cuban Revolution, and waged an urban guerrilla war robbing banks and businesses and seeking to install a Marxist-style government here. He spent almost 15 years in prison.

His running mate, Danilo Astori, is the buttoned-down former finance minister under Mr. Vázquez who shares credit for the macroeconomic policies that improved Uruguay’s social conditions after a financial crisis at the beginning of the decade.

Mr. Mujica, who has piqued the Argentine political establishment by criticizing President Cristina Fernández de Kirchner, said in an interview Sunday on Argentine television that he would “fight hard to have a good relationship with Argentina.”