Johannesburg  - South African opposition leader Helen Zille warned voters Thursday night not to be so "stupid" as to give the ruling African National Congress (ANC) another two-thirds majority in next week's vote, saying to do so would endanger the constitution.

The ANC won 69.7 per cent of the vote in the last elections in 2004.

Despite a split in the party last year and a series of corruption scandals involving party members, polls show the ANC is poised for another comfortable win in the April 22 national and provincial elections, the fourth to be held since the end of apartheid.

A two-thirds majority in the National Assembly gives the party the power to push through changes to the constitution on its own.

Zille, whose DA aims to improve on its 2004 tally of 12 per cent, is urging voters to block that power for fear party leader Jacob Zuma will use it to muzzle institutions of democratic oversight.

Earlier this month, state prosecutors abruptly dropped corruption charges against the ANC leader in connection with a state arms deal, even while admitting they had a strong case.

The chief prosecutor said the case had been damaged by political meddling during the previous administration of Thabo Mbeki, but analysts said the decision smacked of pressure from the ANC.

Zuma has since made veiled threats about the judiciary, warning Constitutional Court judges they are "not God." The ANC government has also recently used its muscle to dismantle the high-performance investigating unit that pursued Zuma and to fire a independent-minded top prosecutor.

"If people are that stupid to give a government like that 66 per cent, two-thirds," they would get the government they deserve, Zille, said, receiving a leadership award from the Rotary Club in Johannesburg.

"Our big revolution in South Africa was not the seizure of power by a liberation movement, it was the day we passed our new constitution that will hold this government to account," she said.

Zille, who is also mayor of Cape Town, held up Zimbabwe as an example of a country where voters turned a blind eye to the former liberation party (Zanu-PF's) anti-democratic slide until it was "too late."(dpa)

But Zille, whose party is seen by blacks as the party of white and mixed-race voters, also admitted it was difficult to impress upon South Africans the need to use their vote to hold politicians to account.

While dissatisfaction with government's record on unemployment, crime and corruption is high, loyalty to the ANC is also still paramount among the black majority.

The DA hopes to prise away the Western Cape province, which includes Cape Town, from the ANC, if necessary with the support of the Congress of the People, a new party of ANC dissidents.