By Noe Torres
MEXICO CITY (Reuters) - Thousands of people marched through Mexico City on Sunday to denounce the July 1 election of Enrique Pena Nieto as president, though the protest was smaller than one held earlier this month.
Pena Nieto's capture of the presidency for the Institutional Revolutionary Party, or PRI, has been challenged by his rival, leftist runner-up Andres Manuel Lopez Obrador, who alleges the PRI resorted to vote-buying and money laundering to win.
Lopez Obrador is seeking to invalidate the PRI victory at the federal electoral tribunal, and the former mayor of Mexico City has pledged to keep the pressure up on Pena Nieto with rallies around the country starting at the end of this month.
On Sunday, student groups dominated the crowd that marched to the capital's main square, chanting slogans like "Pena Out," "Fraud, Fraud" with banners decrying what the protesters called the "imposition" of the PRI candidate on Mexico.
"The people have woken up. The people realize that the PRI violated the elections," said Luis Martinez, a 25-year-old engineering student from Mexico City.
The PRI ran Mexico for 71 straight years until it was ousted in a 2000 election. The party's rule was marred by allegations of corruption, vote-rigging and violent repression of dissent.
Officials in the capital estimated about 30,000 people turned out for Sunday's protest, less than half the number seen at an anti-Pena Nieto demonstration there on July 7.
Authorities said the protest was peaceful.
Separate, smaller demonstrations against Pena Nieto took place in other parts of Mexico on Sunday.
The PRI has dismissed Lopez Obrador's claims and accused him of being a sore loser. The 58-year-old also fought the outcome of the 2006 election, which he narrowly lost to President Felipe Calderon of the conservative National Action Party (PAN).
In 2006, the margin of Lopez Obrador's loss to Calderon was less than 250,000 votes. This time he lost by over 3 million votes to Pena Nieto, who is due to take office in December.
The country's electoral tribunal must rule on Lopez Obrador's challenge by early September. Analysts do not expect the election result to be overturned.
In 2006, Lopez Obrador claimed he had been robbed and declared himself the rightful president of Mexico, leading massive street protests that choked the capital for weeks.
He remained a bitter opponent of Calderon, though this week PAN party chairman Gustavo Madero lent his support to Lopez Obrador's money laundering accusations against Pena Nieto.
Pena Nieto has said he will be respectful of the protesters provided they are peaceful and don't break the law.
(Writing by Dave Graham; Editing by Cynthia Osterman)