Critics say legislation before MPs poses threat to freedom of expression and will put too much power in government hands

Protesters are calling it "Black Tuesday", the darkest day for South African democracy since the end of apartheid in 1994.

Members of parliament will on Tuesday almost certainly pass what is officially called the protection of information bill, otherwise known as the "secrecy bill".

The African National Congress (ANC) says it needs to update apartheid-era legislation safeguarding "valuable information by all organs of state", and has been emboldened by the revelations of wrongdoing at the News of the World and the crisis of press regulation in Britain.

But opposition parties, civil society groups and the media warn of an existential threat to freedom of expression, a pillar of the progressive constitution drawn up under the then president, Nelson Mandela. The Congress of South African Trade Unions, usually an ANC ally, claims the bill puts too much power in the hands of government. An editorial in the bestselling Sunday Times said: "The South African public will be deprived of the vital oxygen of free information."

A war of words has raged between the two camps for more than a year. On one level, the vigorous debate suggests a teenage democracy in rude health. On another, the anger and insults have exposed an ugly mass within South Africa's body politic.

The ANC has dominated party politics for 17 years, gaining nearly double the votes of all its rivals combined. The media enjoys probably greater freedom than any other country in Africa and is seen by some as an unofficial opposition. South Africa's energetic and vocal civil society movements are seen as vital in providing checks and balances to the distribution of power.

For years newspaper readers have feasted on a diet of corruption and financial scandals. The ANC is now attempting to muzzle the press, critics argue, so the powerful can line their pockets with impunity. The new laws would make it a crime to leak, possess or publish information deemed classified by the government, with whistleblowers and journalists facing up to 25 years in jail.

A government source told the Guardian that recent events in Britain had given the ANC confidence to override the storm of opposition. "Phone hacking showed that not even Britain can make press self-regulation work," he said. "We find it very telling that no South African editors have come out and said they never hack phones."

The bill is likely to be steamrollered through parliament, although it must then go to a national council of provinces, and survive court challenges, before being signed into law by the president, Jacob Zuma. The government has accused opponents of spreading misinformation and "sensationalising of the highest order". But activists have vowed to fight to the bitter end, with the Right2Know Campaign organising pickets around the country on Tuesday.

Some say the move to silence journalists is reminiscent of the pre-1994 white minority government. On Sunday the writer Nadine Gordimer, a Nobel prize-winner, said: "The ANC is taking South Africa back to the suppression of free expression of apartheid."

The National Press Club has urged protesters to wear black clothing or a black ribbon or armband on Black Tuesday – the name of the campaign refers to Black Wednesday on 19 October 1977, when the apartheid government banned two newspapers and 19 black consciousness movements following the death in police detention of the activist Steve Biko.

But drawing equivalence with the apartheid regime is highly incendiary in a country where memories are still raw. The ANC's chief whip, Mathole Motshekga, said: "The only result this unfortunate comparison and the planned campaign, in which people are urged to dress in black, will achieve is to dilute the real history of the Black Wednesday and insult the victims of apartheid's barbaric laws."

He added: "The reality of South Africa's vicious history should teach all of us never to campaign in a manner that trivialises the deep pain and suffering experienced by the majority of our people."

A national conversation about what the republic holds sacred is in danger of sinking into a shouting match. The ANC accused Ronnie Kasrils, the party's former intelligence minister, of "ravings" and trying to rule "from the grave" after he criticised the bill. He responded: "As someone who fought in the struggle, I am appalled. I want to raise my voice for the principles the ANC stood for and that is exactly what I've been doing, though it's akin to talking into a toy telephone."

The state security minister, Siyabonga Cwele, rejected calls to include a "public interest" clause to allow for the revealing of secrets, saying: "The foreign spies continue to steal our sensitive information in order to advantage their nations at the expense of advancement of South Africa and her people."

The Sunday Times commented: "These are the sorts of utterances which [the Zimbabwean president] Robert Mugabe has made as he has steadily eroded the freedoms of his people."

In what could be seen as a foreshadowing of battles to come, Zuma's spokesperson, Mac Maharaj, on Saturday laid charges against the Mail & Guardian newspaper and two of its investigative reporters after it ran a story, censored owing to legal threats,, linking him to a shady arms deal. Maharaj also cited the News of the World to make his case. The Sunday Times then alleged that Maharaj and his wife received millions from a French arms company, prompting calls for him to resign.

The secrecy bill is separate from another ANC proposal that has raised concerns – the possible creation of a tribunal that could discipline journalists, with powers to punish that have yet to be spelled out.