Supporters of the Mexican left-wing presidential candidate Andres Manuel Lopez Obrador pitched tents in the middle of Mexico City's Reforma boulevard, blocking traffic.
Thousands of people are taking part in the protests, after Mr Lopez Obrador called on his supporters to paralyse the capital.
Protesters have vowed to continue their campaign until the 31 August deadline for a court ruling on a recount.
The blockades demonstrate the former mayor can mobilise thousands to support him.
Mr Lopez Obrador joined his supporters, sleeping in a tent overnight. He said the protest was not easy, but was "essential for our cause".
People of all ages have joined the protest. While some slept in tents, others grabbed spots outside on the ground, huddling under strips of cardboard and blankets.
There are fears the blockade will cause chaos to the city, which is home to some 20 million people.
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