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Supporters of Mexican presidential candidate Andres Manuel Lopez Obrador attend a daily rally and speech in the Zocalo plaza in Mexico City on Tuesday. ((Gregory Bull/Associated Press))

Mexican officials cracked open ballot boxes Wednesday and began a partial recount of last month's disputed presidential election as leftist protesters continued an aggressivecampaign of civil disobedience.

The recount of ballots collected at nearly 12,000 of the country's 130,000 polling stations is expected to take five days.

The top electoral court ordered the recount last week, ruling that there was sufficient evidence of reported irregularities at about nine per cent of the polling stations.

When the ruling was handed down, Andres Manuel Lopez Obrador of the leftist Democratic Revolution Partyurged his followers to remain calm but promised more acts of civil disobedience.

Obrador's followers have occupied the capital for more than a week, hoping to push their demands for a full recount. Protesters have blockaded streets and pitched tents in the city's main square.

Obrador lost the July 2 election to Felipe Calderon and his National Action Party by less than 0.6 per cent, or roughly 240,000 of the 41 million ballotscast.

Calderon has denied Obrador's charges of fraud and corruption.

Demonstrators gathered outside the headquarters of two banks on Wednesday, as the protest moved outside the city's main square and roads.

On Tuesday, protesters rallied outside the Agriculture Department building, blocking its entrance. Others took over highway toll booths, waving motorists into Mexico City without charge.

Obrador spoke to tens of thousands of people at a Tuesday rally, urging them to remain strong.

"We are going to carry on our struggle," he said. "We are sure we will triumph."

The Federal Electoral Tribunal must declare a president-elect by Sept. 6 or annul the elections.

With files from the Associated Press