Thousands of angry voters marched through Mexico City on Sunday to protest against the "imposition" of Enrique Pena Nieto as the country's new president.
Protesters carried signs accusing presumed president-elect Mr Pena Nieto of electoral fraud and Mexico television giant Televisa of being a "factory of lies."
Opponents say that his Institutional Revolutionary Party (PRI) bought votes and favourable media coverage to win the July 1 election.
It denies the charges and on Friday accused losing leftist candidate Andres Manuel Lopez Obrador of trying to "disqualify the entire electoral process with lies."
Televisa also denies being paid for positive coverage.
In 2006, after Lopez Obrador narrowly lost to President Felipe Calderon, he marshalled hundreds of thousands of supporters to block Mexico City's main centre for weeks.
He has said he will not mobilise people to the streets this time. The 2006 action was highly unpopular with residents.
Student movement I Am 132 has planned a series of events for the coming weeks, starting with Sunday's march, designed to overturn the vote results.
Other groups have said they will blockade the December 1 inauguration of Mr Pena Nieto, who won the presidential election by 6.6 percentage points according to the official count.
The PRI ruled Mexico for 71 consecutive years between 1929 and 2000 - a period notorious for massive corruption, patronage, vote fraud and human rights abuse.
The final vote count must be certified in September by the Federal Electoral Tribunal.
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