CAPE TOWN — The results of South Africa's first free election was tampered with to boost the tally of three opposition parties, including apartheid's last rulers, excerpts of a book published Sunday said.

"Birth: The Conspiracy to Stop the '94 Election", by former electoral commission official Peter Harris, details the untraceable breach as South Africa waited on the edge of violence as the poll results trickled in.

"The hacker went in between 05:56 and 06:41 on the morning of 3 May and made changes to the vote count of three parties," Harris quotes a forensic investigator as saying in the excerpts published in the Sunday Times.

The right-wing Freedom Front's tally was pushed up by between 2.5 percent and four percent, and the white minority National Party which had ruled apartheid South Africa since 1948 rose by around three percent.

The Zulu majority Inkatha Freedom Party (IFP), whose supporters had bloody clashes with frontrunner the African National Congress (ANC), went up between four and five percent.

Election officials froze the results, with Harris in his book pointing to fears of violence and swirling rumors as South Africa steered toward democracy under iconic ANC leader Nelson Mandela.

"There is a real concern that with every hour that the country does not receive election results, the potential for violence increases," he wrote, saying reports of violence were coming in.

Newspapers reported people rushing to stock up on foodstuffs and of farmers clustering in fear of being driven off their land by black militias.

"The frozen election results place the country in limbo. Fear feeds on fear and the political mercury rises. The spooks tell us that security forces have been placed on high alert," he wrote.

South Africa was rocked by bloodshed in the run-up to the polls with one study putting political deaths at 3,000 per year from 1990 when the ban on liberation parties was dropped and Mandela was freed from 27 years of apartheid jail.

Much of the violence was between the ANC and the IFP, which in 1991 the apartheid state faced accusations of backing to fuel the rivalry, which only joined the ballot list days before the country went to the polls.

Deadly last-ditch attempts were also launched by white supremacists to stop the all-race vote from April 26 to 29, with administrative problems pushing voting into a fourth day. The polls drew an 86 percent voter turnout.

The final results were eventually announced as "substantially free and fair" on May 6 with Mandela's ANC winning 62.6 percent of the vote, the National Party 20.4 percent and the IFP 10.5 percent.

Mandela was sworn in as president four days later and served one term.

The next elections in 1999 saw the ANC boost its overwhelming majority by four percent, the New National Party dropping to fourth place, and the IFP receiving 8.58 percent.

Harris's book is due to be released next month.