Johannesburg - The protection of state information bill is flawed and insulting to South Africans, Archbishop Emeritus Desmond Tutu said on Monday.
"It is insulting to all South Africans to be asked to stomach legislation that could be used to outlaw whistle-blowing and investigative journalism... and that makes the state answerable only to the state," he said in a statement.
The bill will come before the National Assembly for a vote on Tuesday.
Tutu said he was appealing to MPs to acknowledge the backlash the bill had caused.
"Please hear the warnings of the academics, civil society leaders, labour representatives, media corps, and legal and Constitutional experts," he said.
"This law will do our people and our country a disservice."
If the bill is passed, the media will not be able to claim it acted in the public interest if it violates or is party to the violation of a law, or publishes classified information to substantiate a report on, for example, malpractice or corruption in government.
"A state is entitled to keep information outside the public domain in instances where publication may jeopardise state security," Tutu said.
"But if the legal mechanism created to protect state security... prevents criminality from being exposed, the mechanism is patently flawed."
Various human rights groups and media houses across the country were expected to protest against the bill on Tuesday.
National Press Club (NPC) chair, Yusuf Abramjee, said the day "will mark the start of a dark day for freedom of speech, freedom of expression and media freedom".
The NPC asked people opposed to the bill to wear black clothes, a black ribbon or a black armband.
It named the campaign "Black Tuesday", based on what became known as "Black Wednesday" - October 19 1977 when the apartheid government banned The World, the Sunday World and a Christian publication Pro Veritas, as well almost 20 people and organisations associated with the black consciousness movement.
On Monday the state security agency (SSA) said claims that the bill would lead to censorship and information blackouts were sensationalist.
"It is not correct that there will be mass classification of information as the application of the bill is narrowed drastically to national security departments," SSA spokesperson Brian Dube said in a statement.
"To argue that life under the protection of state information bill will be characterised by censorship and information blackouts is sensationalising of the highest order."