Brazilian surfing professional Ricardo dos Santos was shot in the back as he walked away from an off-duty police officer who had been on an all-night drinking binge, family members have told the Guardian.

The allegations emerged as thousands of mourners, including some of the world’s top surfers, attended a funeral for the 24-year-old on Wednesday at Guarda do Embaú, near where he was killed on Monday after an altercation with the officer, Luiz Paulo Mota Brentano, and the officer’s younger brother.

“Ricardinho”, as the surfer was best known in his native country, was shot three times after approaching two men who had been creating a disturbance near his home at about 7am on Monday.

Mauro da Silva, an uncle, said the incident started when the surfer and his grandfather asked the man – who was not in uniform – to move his car from an off-road site where he was trying to fix a pipe. The policeman reportedly responded “Quem manda aqui é nós” (We’re the ones in charge around here). Dos Santos replied that it was his neighbourhood and he had never seen them before. Then the man started shooting from inside the car, said Da Silva. “One hit Ricardo in the back, and the other two in his side. He was walking away.” The surfer was airlifted to hospital, but despite four operations and a desperate campaign for blood donations among friends, he died of his wounds on Tuesday.

Brentano is under arrest at Joinsville military police headquarters. He has told investigators he acted in self-defence after Dos Santos threatened him with a machete. This is disputed by family members.

“His grandfather had a pickaxe with him to get at the pipe. But the only thing that Ricardo had in his hands was his phone,” said Da Silva.

Marcelo Arruda, the chief investigator, told the Guardian no knife was found at the scene and no other witnesses – apart from the policeman’s younger brother – reported seeing a knife. Arruda said the policeman and his brother had been drinking and partying so neither of them had slept.

Members of Arruda’s investigative team reported that one of the shots was in the back. But, Arruda said, nothing could be confirmed until the results of the autopsy are finalised.

Police have impounded Brentano’s car and are carrying out forensic tests on his gun. There are also plans for a reenactment.

The surfer’s grandfather, Nicholas dos Santos, who was beside the victim when he was shot, said: “What happened was cowardly.” When the death was announced at the hospital on Tuesday, his mother, Luciene dos Santos, collapsed to the ground, shouting, “You lie, you lie, I want my son back. Bring him back, bring, please.”

Renan Rocha, a close friend of the surfer and his family, described Dos Santos as a “very calm and tranquil person,” although he may have spoken rudely to the off-duty policeman because the officer was drunk after a night out with his friends and was causing a disturbance in a residential area early in the morning. Rocha said several shots missed as well as the three that hit.

Other witnesses allege that Brentano had tried to pick fights with passersby. Brentano had previously been indicted on charges of abuse of authority and causing physical injury, but was not convicted in either case. There are two further cases pending against him that are currently sub judice.

Coronel Claudete Lehmkahl of the military police, told the Guardian it was not unusual for officers on the front line to face multiple accusations, but “that being said, his past does matter and will most likely be added to this investigation”.

For many Brazilians, this reinforces the image of the military police as a brutal carry-over from the dictatorship era. Brazilian police have killed more than 11,000 people over the past five years, averaging about six killings a day – six times more than police in the US, according to the Brazilian Forum on Public Safety, which last year accused police of making “abusive use of lethal force”.

Dos Santos’ Instagram and Facebook accounts contained a desperate appeal made by supporters in his last hours – “Friends, our warrior still needs blood of all types” with the address of the centre in Florianópolis where donations are made. Beside this, thousands of people posted condolence messages. “Rest in peace, shred in heaven,” wrote one. Many also demanded justice. “This worm must not go unpunished,” read one post. “A coward in a uniform interrupts the life of a young warrior”, noted another.

Adriano de Souza, a professional surfer and friend, said he hoped the killing could alert the population. “I hope that from now on, people will demand more justice and this will lead to a change of consciousness. It is increasingly difficult to live in Brazil.”

Kelly Slater, American 11-times surfing world champion, said he had seen surfers paddling out to form a floating memorial to the surfer at Pipeline, the iconic reef break on the North Shore of Oahu, Hawaii. Slater called Dos Santos “truly one of the great barrel riders of his short time”, noting that in 2012-13 he had won Wave of the Winter “for a crazy barrel at Pipeline … was a regular standout in any barrelling or heavy conditions around the world.

“This was truly a senseless loss of life,” he wrote. It unfortunately brings to light the number of murders yearly in Brazil, officially documented at over 50,000 with many thousands more going unreported. Lack of education, poverty, and drugs don’t make for a good mix and make life challenging in this country, one of the most beautiful and scariest places I’ve been.”

Gabriel Medina, the Brazilian world surfing champion and a close friend of Dos Santos, said on Instagram: “Ricardinho, you didn’t deserve this! Never! Why does this happen to good people? I don’t understand.”

He said the surfer, who was known for his fearless barrel-riding skills and won the coveted Andy Irons award for most committed performance at the Billabong Pro Tahiti event in 2012, was a “good person, always helping others, smiling from ear to ear every day, an example to others”.

brett simpson (@BrettSimpo) Will always remember him... So fearless in the water in heavy waves and so kind and gentle on land! Life is so so precious!

Californian Brett Simpson, twice winner of the US Open of Surfing, said on Twitter: “We will always remember him … So fearless in the water in heavy waves and so kind and gentle on land! Life is so precious!”

The South African former professional Travis Logie, Hawaiian Jamie O’Brien and Puerto Rican Brian Toth also posted condolences on social media. “A brother from another motherland,” said Toth. “May you rest in peace brother! You will never be forgotten.”

Billabong issued a statement remembering “a team rider and dear friend, and an inspiration to all who knew him.

“More importantly, Ricardo was a loving brother, son, friend, and grandson. When not traveling the globe in search of giant barrels, Ricardo spent time at home helping around the house and surfing with local groms [kids]. He was also a proud Brazilian who supported his local community. His stoke for his homeland could be felt even as he traveled the globe, telling stories of his time at home with close friends.

“While we mourn this tragedy, we must remember to celebrate and honor Ricardo’s incredible life, and hope it influences the next generation to live a life full of hard work, enthusiasm, and generosity.”

According to surfing magazine Stabmag, Dos Santos had fought to clean up his local streets in the surf community of Guarda do Embaú, saying its relaxed, hippie reputation was being threatened by a boom in the presence of violent street thugs: “Today it seems that this piece of “heaven” is losing its charm, it seems that people no longer value the fact that they are in a beautiful and pure place … it is with an immense feeling of sadness that I write this, but Guarda is being destroyed.”

Additional reporting by Shanna Hanbury and Esther Addley

Facebook Twitter Pinterest 24-year-old professional surfer Ricardo dos Santos has died after being shot by an off-duty policeman near his beachside home in Florianopolis, Brazil.