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HARARE — Zimbabwe's 91-year-old President Robert Mugabe read out the wrong speech at the opening of parliament on Tuesday, an error which the main opposition quickly used to question whether Africa's oldest leader was still of a sound mind.

Mugabe, the only ruler the southern African nation has known since it was recognized in 1980, delivered the same speech he gave on Aug. 25, pinning his hopes on China to help revive Zimbabwe's struggling economy.

Mugabe finished the speech without interruption and his spokesman blamed officials, adding the president would read the correct speech at a later date.

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"The mix-up happened in his secretarial office. Therefore the delivery in Parliament should be set aside," spokesman George Charamba was quoted as saying by the online edition of the government-owned Herald newspaper.

Zimbabwe President Robert Mugabe delivers a speech during the official opening of the third session of the eight parliament of Zimbabwe in Harare on September 15, 2015. JEKESAI NJIKIZANA / AFP - Getty Images

But the main opposition Movement for Democratic Change (MDC), which is critical of Mugabe's long rule, said the blunder called into question his fitness to hold office.

Mugabe shows no visible signs of illness and has denied reports that he suffers from prostate cancer. He has dismissed a fall at Harare airport on April 4 as a simple slip.

"This is a historic blunder. Anyone who is still of a sound mind would have quickly picked it up that the speech was the wrong one," MDC spokesman Obert Gutu told Reuters.

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"But it dovetails with what we in the MDC have been saying that Robert Mugabe is no longer fit for purpose. He should resign," Gutu said.

Mugabe arrived at parliament in a vintage black Rolls-Royce car with his wife Grace, and inspected a guard of honor. The 91-year-old leader was also treated to a fly-past by three fighter jets and to a 21-gun salute.