South Africa's uMkhonto we Sizwe (MK) party will join an alliance of smaller opposition parties in parliament in a bid to take on the African National Congress and Democratic Alliance-led coalition government, it said on Sunday.

The ANC and its largest rival, the white-led, pro-business Democratic Alliance, agreed on Friday to work together in a coalition it called "government of national unity", a step change after 30 years of ANC rule.

Former president Jacob Zuma's uMkhonto we Sizwe party came in a surprisingly strong third in the May 29 election which saw the ANC lose its majority. MK won 14.6% of the vote which translated into 58 seats in the 400-seat National Assembly.

However, MK lawmakers boycotted the first sitting of the National Assembly on Friday after filing a complaint at the country's top court alleging vote-rigging, which the court dismissed as without merit.

Reading a statement on behalf of Zuma, spokesperson Nhlamulo Ndhlela told reporters that the MK party will join the alliance called the "Progressive Caucus", which includes the Marxist Economic Freedom Fighters (EFF) and the centre-left United Democratic Movement.

This alliance commands close to 30% of the seats in the National Assembly, Ndhlela said, sitting next to Zuma - who had a cough but answered questions after the statement - and the leaders of a number of small parties.

"This united effort is necessary because the 2024 election has also resulted in the consolidation of right-wing and reactionary forces who are opposed to economic freedom, radical economic transformation, racial equality and land repossession," he said.

Ndhlela said that MK had decided to take up its seats in the National Assembly after receiving legal advice and that it would continue to raise its allegations of a rigged election in parliament and in courts.

The Independent Electoral Commission has said the election was free and fair.

Zuma also slammed the unity government - which includes two smaller parties, the socially conservative Inkatha Freedom Party and the right-wing Patriotic Alliance - calling it "meaningless" and a "white-led unholy alliance".