Joshua, 10 years Cammarelle's junior and heavily fancied for gold before the final, came from three points down going into the last round to win the final bout of the London Games on the nerve-shredding countback rule.
The Italians opposed the decision, delaying the medal ceremony, but their protest was denied to cheers from an arena packed with partisan support wh o chanted, " Joshua, Joshua, Joshua," as the big Briton finally collected his gold.
"He is an Olympic champion. Once again, there is no easy fight in these Olympics and I pulled it out of the bag," Joshua said.
"It's not just me, it's for my fallen soldiers and the ones that made it and we all drag each other along to the top. So to Team GB, to the 10 of us - three girls and seven boys - congratulations."
Joshua's gold was a fitting end to Britain's best performance in the Olympic ring since 1908, as the men added two golds, a silver and a bronze to the gold won by Nicola Adams in the first women's finals on Thursday.
The hosts finished top of the boxing medals table.
The 22-year-old Britain, one of the tallest men in the competition at 198 centimetres (six feet six inches), bossed the early stages of the first round but Cammarelle landed five big hooks in a row in the closing seconds to take a narrow lead.
Joshua, who came out of nowhere to win a silver medal at last year's world championships, beating Cammarelle in the process, seemed stunned as the second round began and the 32-year-old Italian continued to outfox the young Briton.
But the Italian policeman, twice a world champion at the heaviest weight in amateur boxing, was forced to try and hang on after Joshua launched a torrent of big shots from all angles.
Joshua was able to run 100 metres in 11 seconds when he was 15 and his swift feet showed as, despite questionable defence, he was just about able to level the contest before the 48-45 win on countback brought the house down.
Cammarelle's corner screamed at the judges in disgust as Joshua left the arena waving a British flag, having added to Luke Campbell's gold on Saturday to match the two each won by the men of the boxing strongholds of Cuba and Ukraine.
As he left the medal podium, Joshua hugged former world professional heavyweight champion Lennox Lewis, who had advised him to start throwing more jabs when the pair met at the boxing arena earlier this week.
"The last round showed what heart and courage Joshua has," Britain's Lewis, a dual citizen who won Olympic gold for Canada in 1988, said at ringside. "I don't see anything stopping him right now."
Cammarelle said the score was unfair but magnanimously applauded the crowd as he accepted his silver medal, alongside losing semi-finalists Ivan Dychko of Kazakhstan and Magomedrasul Medzhidov of Azerbaijan who received their bronzes.
"I feel I've lost even though I got a silver medal to go along with the gold in Beijing and the bronze in Athens," the Italian said.