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Pictures posted on social media appeared to show a large part of the building had been damaged.
There has so far been no indication of any casualties and the cause of the blaze was not immediately clear.
A fire swept through the main building of Cameroon's parliament in Yaounde overnight, causing substantial damage but no injuries.
The apparently accidental blaze broke out in the administrative part on the rear facade of the building, the government said, but the debating chamber is believed to have been spared.
The fire was completely out by dawn on Friday, an AFP photographer saw.
"A violent fire devastated the rear facade of the main building of the Ngoa Ekelle Glass Palace last night," Cameroon's public broadcaster CRTV reported, referring to the west African nation's parliamentary building.
The fire broke out around 9:00 pm local time Thursday, and the flames were under control six hours later after causing "a lot of material damage" across four floors but without claiming any casualties.
By the time firefighters arrived, "the fourth and fifth levels had already completely burned out," CRTV reported.
"The fire was already spreading to the upper levels — the sixth and seventh floors — as a helpless crowd looked on."
By Friday morning the rescue operation was over, but some firefighters remained on the scene which police had sealed off.
Images posted to social media showed flames rising several metres as burning debris shot into the night sky above the blaze.
"The auditorium has not been touched," a police officer told an AFP journalist at the scene.
"According to the clues we have today, the cause of the fire is accidental," Communication Minister Issa Tchiroma Bakary told CRTV.
"All available national expertise is being used to assess the damage and tell us precisely the origin of the fire," Tchiroma said, as he thanked rescuers for their work.
"The government has taken, and will take, all measures... for the National Assembly to sit as normal without experiencing any disturbance," he added.
Lawmakers have been meeting this week to discuss the country's 2018 budget.