As firefighters struggled to tame the blaze, a dark plume of smoke and flames rose high into the air above the southern city of Cape Town.
Around 70 firefighters were still battling the fire more than seven hours after it started in the early morning, Cape Town's Fire and Rescue Service spokesman Jermaine Carelse said.
Some of them were lifted up on a crane to spray water on the blaze from above. No injuries have been reported and parliament itself had been closed for the holidays.
Visiting the scene, South African President Cyril Ramaphosa said a person was "being held and is being questioned" by police in connection with the blaze.
"The fire is currently in the National Assembly chambers," Minister of Public Works and Infrastructure Patricia de Lille told reporters as smoke billowed behind her from the roof of the historic white building with grand entrance columns. "This is a very sad day for democracy, for Parliament is the home of our democracy."
"We have not been able to contain the fire in the National Assembly," she added. "Part of the ceilings have collapsed."
JP Smith, the Cape Town official in charge of safety and security, said at least one floor of the parliamentary office building was "gutted" and its entire roof had collapsed. The firefighters were now focusing their efforts on saving the National Assembly building, where South Africa's parliament sits, he said.
Carelse warned that buildings were at risk of collapsing and historical artefacts inside were likely to be damaged or destroyed.
"The bitumen on the roof is even melting, an indication of the intense heat. There have been reports of some walls showing cracks, which could indicate a collapse," News24 website quoted Carelse as saying.
Security guards first reported the fire at around 6am, Carelse said, and the 35 firefighters initially on the scene had to call for reinforcements.
Cape Town activated its Disaster Coordinating Team, which reacts to major emergencies. Police cordoned off the complex and closed nearby roads.
The precinct has three main sections, the original parliament building completed in the late 1800s and two newer parts built in the 20th century.
The fire initially was concentrated at the old parliament building, located behind the National Assembly, de Lille earlier told reporters. During that briefing, she said firefighters "have the situation under control," but the fire soon spread to the current Parliament building.
De Lille said said it was too early to speculate on a cause, adding that authorities were reviewing video camera footage and an investigation was underway.
Ramaphosa and many of South Africa's top politicians were in Cape Town for the funeral service of retired Archbishop Desmond Tutu, which took place on Saturday at the city's St George's Cathedral, about a block away from the parliament precinct.
