The run was almost four seconds off her season's best time and confirmed her as the favourite for the Olympic title.
Viewers compared her running to that of Jamaican sprinter Usain Bolt, saying she was 'making it look like a warm up' and 'just strolled to the finish line'.
Another said: 'I love how Caster Semenya jogs like a boss whilst other contestants sprint on Full Mode.'
But security barred South African reporters who have covered Semenya's career for years from approaching her.
It is believed that only Semenya's coach and team-mates will be given access to her until she leaves Rio next week.
Aware that their heroine is facing an international backlash, there has been outpouring of support from South Africa.
Today, bars and coffee shops were packed with fans anxious to witness the start of her campaign for historic gold.
The 25-year-old has faced objection to her involvement because she has a condition meaning her testosterone levels are far higher than normal.
But she has hit back at 'haters' claiming she should not compete as her fans posted the hashtags #HandsOffCaster and #GoCaster on social media.
Semenya has taken to Instagram to goad her detractors, saying: 'Dear Haters, I have so much more for you to be mad at. Just be patient.'
Semenya has no womb or ovaries - but, and because, of a chromosomal abnormality, she has internal testes.
She was controversially forced to undergo gender testing by the International Association of Athletics Federations following a successful season in 2009.
It was due to a rare condition - 'hyperandrogenism' - which leaves her with testosterone levels three times higher than is expected in women.
Springing to the runner's defence, #GoCaster fans have accused detractors of being sexist and racist.
They have also pointed out the physical advantages enjoyed by the American swimmer Michael Phelps, who has size 14 feet and unnaturally flexible ankles.
During London 2012 Semenya won silver in the 800m after taking suppressants to lower her testosterone levels - on the orders of the IAAF.
But last year, an Indian athlete successfully challenged its mandatory treatment for hyperandrogenism, and the rule was subsequently dropped.
This means that 2016 is the first Olympics that Semenya has competed in without having taken testosterone suppressants.
Since being allowed to compete with unfettered hormone levels, Semenya has clocked some of the fastest times in the world and never looked stronger.
In April she won the 400m, 800m and 1,500m at the South African national championships in just one afternoon.
She also ran her personal best in the 800m last month - leaving her hot favourite for gold in Rio with a possibility of her breaking the world record.
But Semenya's controversial involvement in the competition at all running against other women with normal hormone levels has split opinion.
Ex-British star Paula Radcliffe said her inclusion might encourage other countries to seek out female competitors with high testosterone levels to compete.
World record holder and BBC pundit Radcliffe also said her inclusion compromised the value of the 800m.
