The protection of state information bill will allow government officials to classify significant parts of their work as secret

Controversial secrecy laws proposed for South Africa have received widespread public criticism from corruption watchdogs, trade unions and associates of Nelson Mandela during parliamentary hearings this week.

The protection of state information bill would allow government officials to classify significant parts of their work as secret. Whistleblowers and journalists convicted of possessing, leaking or publishing such information could face up to 25 years in jail.

Despite an outcry from civil society groups and journalists, the bill was passed by the South African parliament's national assembly last year and is now under consideration by the national council of provinces before being signed into law.

Eminent figures queued up this week to criticise the so-called secrecy bill during public hearings at parliament in Cape Town.

Mandela's own views on the matter are not known. The country's first black president is 93, deep in retirement and a unswervingly loyal member of the governing African National Congress (ANC). But George Bizos, his friend and lawyer for half a century, warned on Thursday that the bill would have a chilling effect on would-be whistleblowers: "It will take a pretty brave man or woman to make something public in the public interest when there is uncertainty whether he or she is going to serve a long term in prison," said the veteran human rights lawyer.

"To expose corruption and have to serve this sort of term in prison is undemocratic, it is unreasonable and the courts will not give effect to it. There will be a never-ending queue waiting to take this bill to court if it is passed in its present form."

The Nelson Mandela Foundation, which represents his interests and archive, also raised concerns about the legislation. The foundation's Verne Harris warned that one of its clauses would interfere with a previous constitutionally-mandated act and therefore be knocked down by the constitutional court.

Such a challenge could be launched by the Congress of South Africa Trade Unions. "Our lawyers are standing ready," said its general secretary, Zwelinzima Vavi, arguing that the bill offended the constitutional presumption of innocence.

It is "unwittingly" taking the country back to a police state, Vavi said.

There was further criticism from the non-government organisation Corruption Watch and the country's top corruption investigator, public protector Thuli Madonsela, who said the bill would undermine her work and stifle the free flow of information.

The ANC has mounted a publicity counter-offensive this week with newspaper adverts defending the bill and a march outside parliament on Friday. But the event proved a flop, attracting only 20 people, according to media reports.

Lindiwe Mazibuko, parliamentary leader of the opposition Democratic Alliance, said the march "highlights just how disconnected the governing party has become from the overwhelming majority of South Africans. Indeed, the ANC's march today for secrecy is a march against the South African people."