Anti-apartheid veteran Desmond Tutu is making a last-minute appeal to lawmakers to reject a bill he calls an "insulting" attempt to roll back democracy in South Africa.

Parliament is to vote later Tuesday on a bill defining state secrets and making it illegal to divulge them. The African National Congress, which holds a majority of parliament's seats, sponsored the bill, making it likely it will become law.

Tutu says it is "insulting to all South Africans to be asked to stomach legislation that could be used to outlaw whistle-blowing."

Critics donned black and staged protests at ANC headquarters Tuesday, saying the bill is open to abuse because officials can broadly interpret its "national interest" justification for keeping information secret.