The supporters of leftist presidential candidate Andres Manuel Lopez Obrador, who narrowly lost the election to conservative candidate Felipe Calderon, claim the July vote was rigged.
The protesters surrounded Mexico City offices of U.S.-based Citigroup's Mexican unit Banamex, the Bancomer bank owned by Spain's BBVA and the British company HSBC. They have disrupted Mexico City for 10 days by setting up tents around the business district, and promised to launch more surprise nonviolent protests throughout the week.
During his campaign, Lopez Obrador pledged to reopen the books on a controversial $100 billion bailout of struggling private banks by the government in the mid-1990s, according to the Associated Press. Most of the big banks that survived the crisis were later sold to foreign financial companies.
"We are going to start a movement for the transformation of the nation's institutions," Lopez Obrador said, Reuters reported. "We are going to transform our country, and this is going to happen one way or the other."
Calderon, however, told a gathering of his party's lawmakers, "The solidness of our institutions has overcome the attacks of anti-democrats, anarchists and intolerance."
Election officials plan to recount votes from 9 percent of polling stations in an attempt to clear up allegations of fraud, but Lopez Obrador and his supporters want all 41 million votes recounted.
If the partial recount shows Lopez Obrador securing many more votes, it could force the electoral court to reopen more ballot boxes.
Calderon's margin of victory was about 244,000 votes, or 0.58 percent, reported the AP. His team accuses Lopez Obrador and his supporters of trying to win on the streets what they lost in the polls.