Harare (AFP) - Zimbabwe's 91-year-old President Robert Mugabe read the wrong speech at the opening of a new session of parliament on Tuesday, repeating an address he gave to the legislature last month.

The veteran leader read the 25-minute speech through to the end, apparently unaware that he was delivering the same text he presented during his state of the nation address last month.

"There has been a mix-up of speeches resulting in a situation where... the president delivered the wrong speech," said presidential spokesman George Charamba.

"The mix-up happened in his secretarial office," Charamba was quoted as saying in the state-owned Herald newspaper's online edition.

"The error is sincerely regretted and corrective measures are being considered."

Mugabe, Africa's oldest leader, earlier this year fell down the steps leading from a podium. He was unhurt, but video of the fall went viral on social media.

The opening of parliament was also tarnished by claims by opposition legislators that they had received anonymous death threats warning them against booing Mugabe during his address.

Last month, they booed and heckled him during his state of the nation address in parliament -- which is the speech he repeated on Tuesday.

Movement for Democratic Change (MDC) chief whip Innocent Gonese said seven opposition lawmakers received SMS text messages on their mobile phones warning them not to disrupt Mugabe's address.

"The message is coming from a number which is not reflecting but it's titled 'death'," Gonese told journalists after Mugabe's speech.

"It warns the members concerned to know that immunity ends at parliament and once they step out of parliament that parliamentary immunity does not operate."

Gonese said the party, led by Morgan Tsvangirai, is "worried" about the threats to its lawmakers.

The MDC in a statement called on Mugabe to resign over the blunder, saying it showed that "he is too old."

"This clearly goes to show that Robert Mugabe no longer has the requisite mental faculties that are needed for him to continue in office as the Head of State," said party spokesman Obert Gutu.

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"The MDC would like to call upon Robert Mugabe to immediately tender his resignation as the President of Zimbabwe."

Gutu said Mugabe "is way past his prime" when he failed to recognise that he was reading the wrong speech in parliament.

"This is a sure sign of senility and grossly failing mental and physical health," he said.

"Zimbabwe doesn't deserve to be run by a nonagenarian President who, in fact, is long overdue for retirement."