Tel Aviv, Israel -It starts with a simple knock ... then another, and another. For many on Israel's beaches, it's the sound of a popular sport, while for others it turns the seafront into a battlezone.

Those trying to relax in the sun and sand just find it annoying -- or even dangerous when a game grows too intense.

Matkot, a kind of beach paddleball without rules or points, is the most popular game on Israel's beaches, with fierce, hard-hitting sessions played on the Tel Aviv waterfront and shores across the country.

It starts with the distinctive knock of the hard rubber ball against wooden or carbon-fibre paddles. 

"The sound of matkot is the sound of Israel," said Jordan, a bather who spread her towel close to the action on the beach in Tel Aviv.

"We have come back from a long stay abroad and we really missed that sound."

Her spot right at the water's edge on the eastern Mediterranean is invaded most weekends by hundreds of players because of the firmer surface provided by its damp sand.

Men and women of all ages, athletic or nursing paunches, face off about 15 metres (yards) apart and swipe at the ball, at times with full force, using paddles a little larger than a table tennis bat.

There are no lobs or passing shots here, just the occasional lunge to reach a wayward return.

In general, the body stays immobile and the feet planted firmly on the sand with the arms doing all the work, like trying to swat flies.

There is only one rule: hit the ball straight at your opponent as many times as possible without it falling to the ground.

Locals have been playing this game "for almost 100 years", said Amnon Nissim, 71, a frail figure in baggy white shorts known as "the matkot king".

The evolution of the game and origins of its name are uncertain.

Nissim is aware of similar games on the beaches of California and Brazil as well as elsewhere in the Mediterranean but he insists Israel's matkot is unique and an important part of its culture.

"This is our national sport," he said.

"Criminals!" is the cry from two young Israelis who have launched an offensive against matkot with a tongue-in-cheek documentary, "Matkot - The End" (www.youtube.com/watch?v=pPZO3XX0kJs).

In the short film, an angry woman tells of losing her teeth from an accidental matkot blow.

Co-writer Liran Goldberg told AFP over the phone from a holiday in Denmark, far from the matkot that so annoys him on Israeli beaches, that he had no illusions: the game is here to stay.

- 'Like a bomb' - 

At his home, converted into a matkot museum, Nissim picked up a bat made of carbon that he said sends the ball shooting off "like a bomb".

Two manufacturers in the Tel Aviv area share the market for unbreakable carbon-fibre bats. 

Locals swear by them and are willing to pay up to 200 euros ($225) for a pair, sneering at cheaper imports.

"It's a game that gives a lot of energy to the body, a streaming, flowing game," said Omri Paz, 21, who plays on the beach every weekend.

"This game allows you to disconnect from everything. You are together with your partner and give as much as possible of yourself."

Non-participants are not always impressed.

Apart from the irritating sound, passers-by have to stay alert to swinging bats and keep out of the path of speeding balls.

"Those who don't play, it's a nightmare for them, yes, and they need to go on another beach," said Nir Novak, a tall athletic redhead with a mighty smash.

In 1968, the interior ministry set up matkot-free zones for bathers within a 75-metre (75-yard) radius of lifeguard stations.

Municipal inspectors can issue fines to players who ignore the invisible borders.

"We have always had inspectors roaming the beaches," municipal spokeswoman Mira Marcus told AFP.

"If there is a problem with the matkot playing, the inspector addresses it, and I must say that usually the players accept this and do not want to disturb anyone."