Associated Press reporters on the scene estimated there were about 7,000 protesters.
Police put up the barricades after a 14-party opposition alliance asked its supporters to converge on the commission's heavily guarded office, a police official said. Several people were injured, the official said on condition of anonymity due to policy.
Authorities deployed more than 6,000 security forces, who closed roads around the election office, causing huge traffic snarls in parts of the city of 10 million people.
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