Opposition parties set up a Coalition against Corruption in Durban on Friday, urging voters to resist the Protection of State Information Bill.
Party leaders said the ruling African National Congress intended using the bill to suppress reporting on corruption.
The proposed legislation was similar to laws the apartheid regime used to silence its critics, Congress of the People leader Mosiuoa Lekota said.
African Christian Democratic Party leader Kenneth Meshoe told supporters at the Pinetown Civic Centre: "The looting of state coffers must come to an end."
Also attending the event were the Freedom Front Plus, the Democratic Alliance, the Inkatha Freedom Party, the Azanian People's Organisation, the United Democratic Movement, and the United Christian Democratic Party.
Meshoe said the ANC was not invincible and would be replaced.
"We want a new government of integrity."
Lekota said that without unity among the opposition parties, there would be no opportunity "to make a dent in the ruling party".
He said the launch of the coalition and subsequent meetings across the country were aimed at showing South Africans that unity was possible.
DA parliamentary leader Lindiwe Mazibuko said opposition parties were united in opposition to the information bill.
"When power makes our leaders arrogant, free speech will remind them of their limitations," she said. The bill, if enacted, would limit free speech.
Turning to axed ANC Youth League president Julius Malema's court appearance this week on a charge of money-laundering, she said there had long been questions about his financial affairs.
"He would not have been brought to book if he was still in Jacob Zuma's inner circle," she said.
FF Plus leader Pieter Mulder said that Malema was "a product of the ANC propaganda factory".
"He was brought up on irresponsible ANC propaganda and in a culture of arrogance and self-enrichment."
Mulder said the central issue facing the country was how the current government was spending taxpayers' money.
IFP leader Mangosuthu Buthelezi said that the information bill was a clear indication that an elected government could infringe on the rights of its people.
He accused the ANC of representing the "will of spies" and not the will of the people.
"We know that secrecy and corruption rob our country of service delivery. We are saying no to secrecy, and no to corruption," Buthelezi said.