Images from a video showed a column of black smoke filling the sky, with huge flames billowing from the roof of the building in South Africa.
President Cyril Ramaphosa he visited the site and called it a “terrible and devastating event”.
A 51-year-old man was being detained and questioned by authorities, police said.
The fire, which started shortly after 06:00 local time (04:00 GMT), occurred the day after Archbishop Desmond Tutu’s state funeral at St. George’s Cathedral, near Parliament.
Ramaphosa said the news of the fire was a “terrible setback for what we were enjoying yesterday,” adding that Archbishop Tutu would also have been devastated.
He said the building’s sprinkler system had malfunctioned and praised the firefighters for responding to the fire in minutes. Without his intervention, the National Assembly would have been reduced to “ashes,” he said.
Dozens of firefighters battled the flames. Officials later said that the entire parliamentary complex was badly damaged.
Jean-Pierre Smith, member of the security and protection committee of the mayor’s office Cape Town, He told reporters that the roof over the old assembly hall had “completely disappeared” and that further damage inside the old chamber had not yet been assessed.
“It is not possible to see if it is damaged. We hope it is not because it has so many historical artifacts, but you cannot access it without breaking the doors and we do not want to do that,” he said.
He also said that the parliament’s fire alarm only sounded when firefighters were already on the scene.
Authorities said the fire started in the third-floor offices and quickly spread to the House of the National Assembly (the lower house of parliament), reports the BBC’s Nomsa Maseko in Cape Town.
No injuries have been reported and there is no indication of what could have caused it, adds our correspondent.
Fire and rescue service officials said it could take several more hours to put out the fire completely due to the building’s carpets and hardwood floors.
Damage caused by the fire can be seen in images provided by the city of Cape Town.
Parliament is not currently in session due to the holidays.
The Houses of Parliament in Cape Town are made up of three sections, the oldest dating from 1884.
The newer sections built in the 1920s and 1980s house the National Assembly. Meanwhile, the government is headquartered in Praetorship.
Second fire in South African parliament
It is the second fire in the parliament of South Africa in less than a year. In March there was a fire caused by an electrical failure.
Last year, a fire devastated part of the Cape Town University Library, which housed a unique collection of African archives.
In AmericanPost.News we will follow the latest news from South Africa as it “outgrew” the Omicron variant and canceled the curfew.
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