Renewed protest greeted the annual nine-day event.
The first running of the bulls is scheduled for Tuesday.
The festival has been held in the city of 215,000 in the Navarre region since the end of the 16th century, always in July.
"San Fermín, San Fermín, San Fermín," the closely packed throng chanted, as they danced, sang and waved the traditional red scarves.
Six 600-kilogram fighting bulls will be driven through the narrow streets of the Old City every morning up until July 14, with hundreds of people, most of them young men, running ahead of them.
The event is broadcast live by several broadcasters.
Dozens of people are injured every year over the course of 875 metres.
There have been 16 fatalities since 1924.
In the evening, the same bulls are killed in bullfights in the city's bullring.
Other festival events include concerts and processions.
Animal rights activists have protested against the running of the bulls for years, terming it "medieval torture".
People come from all over the world for the event, mainly from Europe, Australia, Asia and the United States.
US author Ernest Hemingway used Pamplona and the running of the bulls as the backdrop for his first major novel The Sun Also Rises in 1926.
It was published in the United Kingdom the following year under the title Fiesta.
