The observers from the European Union (EU) submitted their report to Mexican officials on Saturday, saying Mexico's July 2 presidential elections were transparent, lawful and legitimate.
Jose Ignacio Salafranca, head of the EU observers' mission, said there were irregularities but the vote results were valid.
"These elections were free, authentic and the results represent the legitimate will of the Mexican people," Salafranca said in a statement distributed by Mexico's Attorney General's Office.
"The elections complies with respect for the law, transparency and the principles of the rule of law," he said.
Felipe Calderon, of the ruling National Action Party, won the July 2 presidential elections and will take office on Dec.1.
However, the result of the presidential elections was contested by defeated presidential candidate Andres Manuel Lopez Obrador, of the left-wing For the Good of All Coalition, who was sworn in as Mexico's "legitimate president" on Monday in a move aimed at keeping President-elect Calderon from assuming office.