The man was arrested inside parliament and the case has been handed over to an elite police unit called the Hawks, public works and infrastructure minister Patricia De Lille said.

Flames could be seen and a column of smoke rose into the sky early on Sunday morning, beginning at about 5.30am local time (3.30am in the UK).

The blaze started in third-floor offices and spread to the National Assembly chamber, local fire service spokesman Jermaine Carelse said, adding that no one had been injured.

It is feared that parts of the complex, some of which date back to 1884, might collapse because of the heat.

President Cyril Ramaphosa said it was "devastating news" but the work of parliament would continue.

Speaking to reporters, he said: "I believe somebody is being held right now and they're being questioned."

He added: "It does seem like the sprinkler system did not work as it was supposed to, but their (firefighters') appearance and their coming here has saved a very important national asset of our government."

Some 60 firefighters have been on the scene tackling the blaze.

Ms de Lille added that someone had closed one of the valves on the sprinkler system, so there was no water to trigger it coming on.

"We can't tell you exactly where the fire started," she said, adding that it was a "very sad day for our democracy".

Jean-Pierre Smith, a Cape Town mayoral committee member responsible for safety and security, said the roof of the old building had collapsed and the third floor had been gutted, including office space and the gym.

He also told journalists that the parliament's fire alarm rang when firefighters were already on site.

President Ramaphosa and many of South Africa's high-ranking politicians were in Cape Town for the funeral service of Archbishop Desmond Tutu, which took place on Saturday at the city's St George's Cathedral, about a block away from the parliament precinct.