Enrique Pena Nieto has won Mexico's presidential election by 3.3 million ballots, or almost seven percentage points, electoral officials said.
But the results, announced on Sunday by the electoral tribunal, also cited allegations of excessive campaign spending and voter fraud during the July 1 ballot.
According to the final count, Pena Nieto got 19.2 million votes, or 38.21 per cent, compared to 15.9 million, or 31.59 per cent, for second-place finisher Andres Manuel Lopez Obrador.
In third place with 12.8 million votes, or 25.4 per cent, was Josefina Vazquez Mota of the National Action Party, or PAN, of President Felipe Calderon, who was barred by the constitution from running for a second term. The PAN was hurt by a sluggish economy and dissatisfaction over drug-related violence.
Calderon has congratulated Pena Nieto on his election win, as have leaders from dozens of other countries.
Ruben Olmos, the director of a Washington-based consulting firm called Global Policy Strategies, said that Pena Nieto cannot take office until the Electoral Tribunal gives him a "diploma" that officially names him the president of Mexico when incumbent Calderon completes his term on November 30.
Mexican tribunals have until September to rule on any wrongdoing and officially name Pena Nieto as president. Both Lopez Obrador and the PAN have alleged that Pena Nieto overspent, although they have yet to file legal complaints to the tribunal.
Olmos said that the "young democracy" had made a lot of progress in recent years and that an annulment was "highly unlikely".
Lopez Obrador said on Saturday that he is gathering evidence for a legal challenge and urged the PAN to join him.
Tens of thousands also marched through the Mexican capital against Pena Nieto on Saturday, holding banners accusing him of being corrupt and authoritarian.
Pena Nieto denies any wrongdoing, and officials from his Institutional Revolutionary Party (PRI) say they could sue Lopez Obrador over his accusations.
Olmos said that it will be a challenging term for Pena Nieto and that he would have to address opposition from groups that have been protesting against him, but the director said that Pena Nieto had begun to do so "by addressing the fact that youngsters are participating in this young democracy".
A minority in Congress
Vote counts also confirmed that the PRI, and its allies in the Green Party, would have a minority in both houses of Congress, which could complicate Pena Nieto's agenda when he takes office in December. Officials projected that the two parties will have 240 deputies in the 500-seat lower house of Congress, which will convene in September.
Pena Nieto, however, may also be able to garner support from 10 deputies from the centrist New Alliance party.
The PAN will have 116 deputies, according to projections. But Calderon's party may also support energy and labour reforms that Pena Nieto has proposed.
The president-elect has advocated allowing foreign investment into the oil industry, and making it easier for employers to hire and fire workers.
In the 128-seat Senate, meanwhile, the PRI and Green Party will have 62 seats, followed by the PAN with 38.
A group of three other parties, who want to keep existing labour protections and national control of the oil industry, have the remaining 134 seats in the lower house and 27 senators in the upper.
The number of deputies and senators could change following rulings by the electoral tribunal, officials said.
Pena Nieto's win returns the PRI to presidential power after 12 years in the political wilderness. It had previously held the presidency continually from 1929 until 2000.