The nine-day street party got underway at midday with the cry "Viva San Fermin!" followed seconds later by the firing of a firecracker known locally as the "chupinazo".
Crowds wearing traditional white outfits trimmed with red neckerchiefs and cummerbunds drank from traditional leather wine pouches and sprayed red wine on others. Others poured wine from balconies overhead.
Five giant outdoors television screens were set up at other points in the city centre and aired the event live.
The festival, immortalised in Ernest Hemingway's 1926 novel "The Sun Also Rises", dates back to medieval times and features religious processions, folk dancing, concerts and round-the-clock drinking.
Just over six tonnes of fireworks will be set off during a nightly sound and light show during the festival which wraps up on July 14.
But the highlight is a daily race of courage pitting hundreds of people against a pack of six half-tonne fighting 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, in on Tuesday. A run takes on average just under four minutes.
