"Now it is crunch time," she said in a speech delivered in Cape Town ahead of a week-long "plane tour" campaign to locations around the country.

The tour ends on Monday next week, two days ahead of the April 22 national and provincial elections.

Her destinations include Butterworth, Bredasdorp, Springbok, Potchefstroom and Polokwane.

According to extracts from Zille's speech sent to Sapa, she said Zuma and his inner circle had already gone far in their determination to entrench their power.

"They have destroyed the Scorpions, they have pressurised the National Prosecuting Authority to drop Zuma's corruption charges and they have engineered the release of their crony, Schabir Shaik, from prison," Zille said.

"Next on their agenda is to remove every other obstacle to unfettered power. Press freedom, the independence of the judiciary and every section in the Constitution which limits the ruling party's power are under threat."

Zille urged supporters to turn out in large numbers to stop a newly-elected African National Congress, under Zuma, from maintaining a two-thirds majority that would allow them to "unilaterally change the Constitution to abuse power".