Ten people were killed, including six women and four men, and 85 others injured, the news agency Ouest-France reported citing a statement by Nyusi's ruling party, the Mozambique Liberation Front (FRELIMO).
The deadly stampede reportedly occurred closer to the end of the event as people rushed to the exits of the overcrowded stadium.
The tragic incident happened ahead of general elections in the African country that are scheduled for 15 October.
Nyusi and his FRELIMO party, which has governed Mozambique for over 40 years will face opposition, including from the right-wing party Mozambican National Resistance (RENAMO). They fought against the government for six years before striking a peace deal and becoming a legal opposition party in August.