Thousands of revellers are seeing red in Spain after facing off in the mother of all food fights.

About 20,000 people from across the globe took up arms, or tomatoes, in the annual La Tomatina festival in the tiny eastern town of Bunol.

In a break with tradition, the tomato turmoil began early after Mother Nature gave the crowd a taste of what was to come.

As rain pelted the participants, the first of the festival trucks entered the packed streets to unload the ripe and juicy ammunition.

Within moments everyone and everything was soaked in a soupy mix of rain water and red pulp.

Locals and tour guides said the rain was a first for the event, which is thought to have originated from a food fight between children during a parade in the mid-1940s.

It was a continuation of firsts for the festival, which introduced an entrance fee and cut participant numbers this year after concerns about rising costs and ballooning attendance.

Tour guide Andrea MacDonald, who was attending her third La Tomatina, said the changes were positive for both locals and partygoers.

She said limiting the numbers had led to a safer experience for all.

"It was much more fun this year.  Last year they had 40,000 people ... and they were pulling girls out who were having anxiety attacks," she said.

Australian tomato-throwers  Kat, Stef and Georgia had been warned about having their shirts ripped in the mushy mosh.

But despite finding themselves in the centre of the action, the trio said the fighting was mostly fair.  

"I had heard so many stories but I did not get hassled at all, " Georgia, from Tasmania, said.

"We picked up tomatoes off the ground and shared them.

"We squished them before we threw them.

"The guys on the trucks didn't, but they're boys."

Participants came dressed in imaginative costumes, and those that left little to the imagination.

Some, expecting an 11am launch, mistook the rocket signalling the start of proceedings for thunder.

But there was no mistaking the rapid fire of tomatoes that followed.

Participants cheered as the trucks returned again and again, parting the now red sea of revellers to unload the precious cargo.

Some rejoiced when they discovered new ammunition in the form of a whole or half tomato, while others sought shelter behind the tarps protecting the locals' houses.

And when there were no more tomatoes, people made do with the pulp and juice, loading up empty plastic sangria and beer glasses to find their next victim.

After one hour the mushy madness came to a close and, with the smell of rotten tomatoes in the air, revellers began to exit the town centre.

"I just can't get the taste of tomato out of my mouth. I don't want to see another tomato in my life," Australian Tain Breedon said.

Portable showers were set up so revellers could wash off the red muck, while others sought the help of locals armed with garden hoses.  

And smartphones became mirrors as people picked remnants of tomato out of their hair.

The town's more entrepreneurial residents offered the use of showers and toilets for a small fee.

As Bunol cleaned up after its one-day invaders, the festival fighters headed back to Valencia on the scores of tour buses.