Dressed in white clothes and red scarves as per tradition, thousands of people filled the Pamplona City Hall square at noon (1000 GMT) to attend the "chupinazo" -- the launch of the firecracker.
This city in northern Spain is awash with red and white for nine days.
The run became world famous after being immortalised by US writer Ernest Hemingway in his novel "The Sun Also Rises" in 1926.
The San Fermin festivities, with medieval origins, also include concerts, religious processions and lots of wine.
The climax, however, comes every day at 8:00 am in when hundreds of attendees launch themselves into a dangerous race of 848.6 metres before six heavy fighting bulls through the narrow streets of the city centre.
The long-awaited first running of the bulls of this year will be held this Sunday, the day of San Fermin.
Dozens of people are injured each year, although most are injuries resulting from falls or being stomped by animals. Deaths have also been recorded.
