A street cleaner aims his hose at a pile of litter in a Pamplona street, sending it into a gutter -- and forcing revellers at Spain's San Fermin festival to flee to avoid getting wet.

"Watch out!" a young man dressed in white with a red scarf around his neck yells out to his two friends sitting on a nearby kerb drinking beer as the stream of water loaded with plastic cups and wrappers approaches.

The northern city of 200,000 residents attracts about half a million visitors during the nine-day festival which wraps up Saturday.

They are drawn by early morning bull runs, late night concerts and round-the-clock drinking and dancing, with bars allowed to remain open during San Fermin until 6 am.

This non-stop activity adds to the challenge faced by clean-up crews tasked with removing the tonnes of litter dropped each day by party-goers on the narrow, cobbled streets of the centre of Pamplona during the annual fiesta.

Some people try to climb into trucks or bend hoses to stop the flow of water.

Clean-up workers sometimes have to physically remove drunken revellers who have fallen asleep in squares in the centre of Pamplona to be able to do their jobs.

"When there are many people in the streets, getting through the crowd is dangerous because the trucks are big," said Karlos Teres, 46, head of a team of eight people who clean the streets of the city every morning.

"Most people move along but then there are the typical drunks who place themselves in front of our vehicles and in the end you have to push people out of the way."

On rare occasions revellers turn violent.

Two men beat up the driver of a truck on July 6, the opening day of the fiesta, causing a fracture to his skull and cuts and bruises to his face that required three days in hospital.

"My colleague became very frightened because I lay on the ground unconscious, surrounded by a pool of blood," Saturnino Perez Monge, 49, told local newspaper Diario de Navarra.

Festival-goers have become more aggressive with clean-up crews over the years, said Luis Hualde, the head of FCC Pamplona, the company contracted to clean the streets of the city and who has worked during San Fermin for two decades.

"This is especially the case at certain hours of the day, early in the morning. I associate it with the consumption of drugs," the 45-year-old said.

Most other festivals last just one day, allowing crews to clean streets once the crowds have left, but at San Fermin "we have to find a way to reconcile cleaning with 24-hour partying," he added.

FCC Pamplona employs 110 people throughout the year to clean the streets of the city. But during the San Fermin fiesta the number of cleaners rises to 250.

The extra staff cost Pamplona city hall 400,000 euros ($488,500), a spokeswoman for the municipality said.

The clean-up crews remove around 60 tonnes of litter from the streets on weekdays -- and up to 100 tonnes each day on weekends when the number of visitors to the city swells.

FCC Pamplona has set up five teams made up of two workers each who focus exclusively on cleaning up places where men tend to urinate.

"These are 30 to 40 spots which we have identified as problematic because they are away from the festival where there are corners where people have the habit of urinating in public and it smells horrible," said Hualde.

The crews use a high-power hose to blast these corners several times a day with water mixed with disinfectants.

The water used by crews to hose down the streets is mixed with a chemical that generates a light smell of lemons.

The company will use half a million litres of water, and 1,500 litres of disinfectants and deodorisers during the festival.