Novak Djokovic's father says moves to deport his son from Australia are politically motivated.
Srdjan Djokovic accused Prime Minister Scott Morrison of trying to bring Serbia "to its knees".
Djokovic, the world number one male tennis player, is currently in immigration detention in a Melbourne hotel after being detained by Border Force.
The 34-year-old Serbian star will remain there until at least Monday when a federal court challenge by his lawyers against the Australian government's decision to deport him will be heard.
Srdjan Djokovic said the decision was an attack on Serbia and its people.
"They're keeping him in captivity," he said during a news conference at his restaurant named after his son in the Serbian capital Belgrade on Thursday.
"They're stomping all over Novak to stomp all over Serbia and Serbian people."
"Serbia has always shown that he [Novak] comes from a proud nation.
"This has nothing to do with sports, this is a political agenda.
"Novak is the best player and the best athlete in the world, but several hundred million people from the West can't stomach that."
Before his flight to Melbourne, Tennis Australia and the Victorian government had issued Djokovic with a medical exemption from being fully vaccinated against COVID-19, apparently clearing him to travel to defend his Australian Open title.
Djokovic landed in Melbourne late on Wednesday evening after a 14-hour flight from Dubai, but was detained by Border Force shortly after arriving.
It was revealed on Thursday morning that his visa had been cancelled as he did not meet entry requirements.
Djokovic 'a prisoner' in hotel
Djokovic's mother, Dijana, said she had spoken to her son, who was having trouble sleeping, and described his treatment as "not human".
"It's just not fair – it's not human.
"I just hope that he will be strong as we are trying also to be very strong to give him some energy to keep on going.
"I hope that he will win."
On Thursday afternoon several hundred people descended on Serbia's parliament building in Belgrade to attend a rally organised by Djokovic's family.
The crowd, many draped in Serbian flags, chanted "Novak" as his mother, father and brother addressed the crowd.
"This is a shameful act, absolutely shameful," one supporter, Sasha, told the ABC at the rally.
"We are hoping that things are going to change, but not much hope for that."
Ms Djokovic also blasted the conditions of the accommodation at the hotel, which she described as "terrible".
"So for me this is really [an] outrage."
A request for him to stay in the rented apartment his team is staying in was reportedly denied.
Djokovic, a nine-time Australian Open champion, was hoping to defend his 2021 crown and become the most successful male tennis player in the Open era, with 21 career grand slams.
He currently sits tied on 20 grand slam titles alongside Roger Federer and Rafael Nadal.
