Politicians, police and members of the public have condemned “horrific” racist graffiti targeting the home of a 10-year-old boy and his father days after they moved in.

Jackson Yamba, 38, said his son was left traumatised and afraid to leave the property after the words “no blacks, no blacks” were sprayed on their door earlier this month. The slogan also appeared on communal doors in the small block of flats in Salford, Greater Manchester, five days after they moved in on 3 February.

Yamba, a former barrister from the Democratic Republic of the Congo, said: “I would expect this in the 70s or the 50s, but nowadays? I don’t think this should be accepted in a 21st century multicultural society.”

Yamba said his son, David, burst into tears when they discovered the “abhorrent” graffiti on their way to the boy’s school on 8 February.

He immediately reported the vandalism to Greater Manchester police, but said he had not received a call or visit more than a week later, which led him to tweet about the incident on Saturday night. His post was shared thousands of times, prompting an outpouring of support from politicians, celebrities and fellow Salfordians.

Speaking to the Guardian on Sunday, Yamba said: “As we opened the door, we saw the graffiti on the door: ‘No blacks, no blacks.’ My son started crying – he’s been totally traumatised. I couldn’t believe what I witnessed.”

Yamba, who said he was the only black person in their block of eight flats , said he had been “appalled” at the lack of response from the police and that his frustration prompted him to speak publicly. Ian Hopkins, the chief constable of GMP, admitted the force’s response was “frankly just not good enough” as a hate crime investigation was launched on Sunday.

Facebook Twitter Pinterest Jackson Yamba and his son, David. Photograph: Twitter

Yamba, a senior paralegal at a law firm, said he had lived in Britain for 13 years and never experienced such racism. The trainee solicitor said he did not know whether Brexit was to blame for a rise in hate crimes, but that it had “opened a Pandora’s box”, with the debate centred on immigration.

He said he was considering moving away from the area for the sake of his son. “He used to go to school alone, but since that day he can’t even go through the corridor by himself. He’s traumatised by the incident,” he said. “Everywhere I go now, he has to be with me. He can’t even stay five minutes on his own because he’s scared. In his mind he thinks whoever has done it can come and harm us.”

David, who started school in Salford in September, said: “Me and my dad were going to school and when we were walking out it said, ‘No blacks, no blacks’ on it, so I was scared and my dad kept telling me to go out.

“I was worried they were waiting for us outside to see our reaction. We went outside and called the police. We started walking down the stairs and we saw more ‘No blacks, no blacks’ and I was scared because I thought, ‘The person who’s done this knows where we live.’ I kept holding on to my dad.

“When I first saw it, I wished we could move, because that’s never happened to me before. I thought the police would come – I wish they would come and catch the person who did it.”

The incident triggered an outpouring of condemnation on social media, with some Salford residents offering to paint over the graffiti or buy the family a new front door. Yamba said his neighbours had also been in touch to offer their solidarity, while the pair have been invited as special guests to the Salford Red Devils rugby club.

“I’m grateful and humble,” said Yamba, as his son played video games in his bedroom. “It has made me feel more confidence in the sense that I’m not out here alone and people will stand with you”.

Andy Burnham, the mayor of Greater Manchester, said he was disappointed in the police’s response to the “truly sickening” act and that the chief constable was right to scrutinise the slow response.

He added: “While this was a truly atrocious and depressing act, the immediate and widespread condemnation from across the community was heartening and reveals the true character of the people of Salford and Greater Manchester.”

Rebecca Long-Bailey, the Labour MP for Salford and Eccles, said: “I am shocked and appalled by this vindictive racist act leaving Mr Yamba and his family living in fear. It is sadly not reflective of Salford and the outpouring of support to Mr Yamba and his family on Twitter demonstrates that Salford and many other places do not tolerate racism. Such racism has no place in our city and it is right that we act swiftly to stamp it out.”