Mr Guebuza – who is the first former president to take the witness stand in a court case in the country – defended his government’s decision to secretly borrow $2bn (£1.48) to finance a fishing fleet.
The power cut out several times during his testimony, which was broadcast live on local television channels, AFP news agency reports.
Mr Guebuza, 79, said he had acted because of a threat to national security. He then told the court not to dwell on defence matters as it was not in their remit.
His son, Ndambi, is one of 19 defendants on trial, accused of diverting $500m from the loan, which was not declared to the Mozambican parliament.
He has denied charges that he took bribes to persuade his father to approve the maritime deal and facilitate the secret loans.
Ndambi Guebuza sat in court dressed in an orange prison uniform, as his father testified, AFP reports.
He has been in detention since February 2019.
When the loans came to light, international donors cut off financial support, sparking a sovereign debt default and currency collapse.
Source: BBC