Mexico City ? Political unrest here is taking an economic toll, rattling investors and costing businesses in the capital millions in lost sales.

Observers here say leftist presidential candidate Andres Manuel Lopez Obrador may be gaining momentum with his calls for a recount of the July 2 balloting. Results showing that he lost by a whisker to conservative, pro-business rival Felipe Calderon have been dogged by allegations of vote tampering and dirty campaigning.

Street protests by supporters of Lopez Obrador, the popular former mayor of Mexico City, have grown in recent weeks. His followers seized control of one of the capital’s main arteries on Sunday, setting up tent encampments following a massive rally in the historic city center to pressure election officials to perform a vote-by-vote recount of all 41 million ballots cast in last month’s presidential contest.

Mexican financial markets sank Tuesday as the blockade that has bottled up one of the capital’s principal thoroughfares entered its third day. Mexico’s benchmark 10-year bond and its currency, the peso, weakened as uncertainty gripped a polarized nation.

“There is the risk that things could spin out of control and become violent,” said Latin America analyst Alberto Ramos, senior economist with Goldman Sachs & Co. in New York.