The nine-day fiesta got underway at midday to cries of "Viva San Fermin!" at city hall, followed seconds later by the firing of a firecracker known as the "chupinazo".
Masses of merrymakers, many wearing traditional white outfits trimmed with red neckerchiefs and cummerbunds, danced and sprayed each other with cheap wine as red and white confetti rained down on them.
The crowds passed large yellow and black inflatable balls over their heads as scores looked down from packed apartment balconies.
The winner was 85-year-old Jesus Ilundain Zaragueta.
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The festival in honour of the patron saint of Spain's northern Navarra region -- San Fermin -- dates back to medieval times and combines religious processions, all-night partying and hair-raising daily bull runs.
Each day at 8:00 am hundreds of people race with six huge bulls, charging along a winding, 846.6-metre (more than half a mile) course through narrow streets to the city's bull ring, where the animals are killed in an afternoon bullfight.
The first bull run, which traditionally draws the largest number of participants, is on Thursday. A run takes on average just under four minutes.
Fifteen people have been killed in the bull runs since modern day records started in 1911.
About 50 semi-naked animal rights activists daubed themselves with fake blood and stood outside of Pamplona's bullring on Tuesday holding signs that read: "Pamplona: Bloodbath for bulls" in several languages.