Thrill-seekers sprinted through Pamplona in a swift and relatively clean start to the running of the bulls.
No one was gored on Tuesday, but four people were hospitalized with bumps, bruises or scrapes, Spanish Red Cross spokesman Jose Aldaba said.
"I feared for my life. It was pretty intense," said 23-year-old runner Mark Kowalski of Edmonton, Alberta.
The six fighting bulls and six bell-tinkling steers — meant to keep them in a tight pack — charged down the 850 meter course from a holding pen to the northern town's bull ring.
Runners, wearing traditional white clothing and red kerchiefs around their necks, tripped over each other or fell after getting bumped by bulls or steers, but apparently no one was seriously trampled. One man went down about midcourse and half a dozen animals raced over him, but he was able later to get up and walk away.
At another point, a bull slipped and fell on the cobblestone street and landed atop a runner. Some of the daredevils rested their hands on the bulls, a gesture considered disrespectful to the animals.
People came from all over the world to test their bravery and enjoy nonstop street parties. Spanish Television said about 2,000 people took part in the first of eight runs at the famed San Fermin festival, made famous by Ernest Hemingway's novel "The Sun Also Rises."
Tuesday's run lasted about 2.5 minutes, which is quick for San Fermin runs.
"It was incredible," said Victor Gaona, a 26-year-old from Mexico. "I saw a bull fall in front of me, and it is an unforgettable experience."
Fourteen people have died at San Fermin since record-keeping began in 1924. The last fatal goring was in 1995. The victim was Matthew Tassio, a 22-year-old American.
In 2003 Pamplona native Fermin Etxeberri, 63, was trampled in the head by a bull and died after spending months in a coma.