Staff at Milpark Hospital declined to speak to journalists who descended on the premises for news on the elder statesman.
By 19:30, the only formal word on the condition of 92-year-old Mandela remained a short statement issued by the Nelson Mandela Foundation spokesperson Sello Hatang around 16:15.
It stated: "We can confirm that Mr Mandela is at Milpark Hospital undergoing routine tests. He is in no danger and is in good spirits."
Hatang did not answer his cellphone for follow-up questions.
Mandela's daughter Makaziwi from his first wife Evelyn said: "You got the statement? No further comment."
Members of the Mandela family were seen coming and going at the hospital by reporters.
A staffer from the French news agency AFP saw Mandela's wife, Graça Machel, leaving the hospital around 17:30, with four other family members departing shortly after.
At a far corner of the hospital, a makeshift barrier of green mesh was erected. Inside the area were VIP cars, presumably belonging to family members.
As night fell over Johannesburg, journalists, photographers and cameramen lined a nearby bridge outside the hospital.
A hospital official was expected to address them but this had not yet happened.
One of Mandela's grandsons was seen buying magazines and refreshments at the hospital eatery. Ndaba Mandela and his youngest brother were also seen near the barricaded area.
Mandela's long-time aide, Zelda la Grange, flanked by a bodyguard entered the barricaded area.
Hospital security remained on high alert, ushering wandering journalists to the bridge.
Some journalists sat inside the hospital waiting area, blending with hospital visitors and patients. One journalist carrying a brown paper bag, was seen limping in an attempt to blend in with the ill.
A hospital nurse standing outside the main entrance was overheard saying: "I saw him being flown in (by helicopter) at 14:00."
Mandela's "routine tests" came a day after Archbishop Desmond Tutu told Sapa that Mandela was "frail".
"I saw him last week," Tutu said in Cape Town on Tuesday.
"He was all right, I mean he's 92, man, you know. And he's frail."
Twitter was abuzz on Wednesday evening with news of Mandela being admitted to hospital.
Sandiso Ngubane, @Sandiso_N, tweeted: "My thing is; why would family members from Qunu come to check Madiba out during his "routine check-up"? Sometimes spin is just that... Spin!"
Other tweets speculating about Mandela's health included Siphojanuary who tweeted: " why would the family and high profile people be visiting #Madiba in hospital if he just went for a routine check-up???!"
RanjeniM tweeted "having dinner at Nelson Mandela Square cos only place I can feel close to him now. That statue is somehow comforting #Madiba".
"Hope Mr Nelson Mandela is okay n it really is a routine check up!! I'm really woRried **praying hard**," wrote another twitter user Purplemooky.
Twitter user Norwin tweeted: "#Madiba not in ICU; family and friends seen chatting and laughing. Hospital security stepped up".
"If something happens to Nelson Mandela tonight or in a day or so, do I close shop and let my staff off for the day? 80 employees nationwide?" user WarrenRSmith said.
Earlier this month, a report circulated on the social network that the elder statesman had died.
It was condemned as malicious and insensitive by the African National Congress.
Afrikaans Sunday newspaper Rapport said though the rumours were false, reliable sources had confirmed that Mandela's health had deteriorated.
In mid-January, a Nelson Mandela Foundation spokesperson said Mandela was well and on holiday with his wife Graça Machel.