Zuma is scheduled to address more than 100000 party supporters and 6000 VIP guests - who include 46 heads of state - at a rally in Mangaung in the Free State today, to celebrate the ruling party's 100th anniversary.

Roadblocks were mounted on major roads leading to the Free State. Buses carrying supporters were searched following rumours about Limpopo youths carrying stones with which to pelt the president. No stones were found.

SAPS spokeswoman Brig-adier Nonkululeko Mbatha said: "We are receiving intelligence and we are able to act pro-actively. We are leaving no avenue open [to disruption]."

In a move seen as an attempt by party leaders to shield Zuma from possible embarrassment in front of international guests, the ANC announced earlier this week that only the president would be allowed to address the crowds today.

Zuma's detractors have alleged that this was an attempt by the leadership to ensure that suspended youth league leader Julius Malema does not get an opportunity to greet the crowd.

But ANC secretary-general Gwede Mantashe rejected that interpretation of the change in programme from recent years.

He said throughout most of its history the ANC had not allowed its leagues and alliance partners, Cosatu and the SA Communist Party, to speak on such occasions. The practice of doing so began after a new party leadership, under Zuma, was elected in 2007, he said.

"This thing of creating space for the youth league is a post-Polokwane practice. You're having a centenary, usually the president speaks at 12, but now he will speak at 5pm. To think that we can run that programme as if we are running last year's January 8 statement ... you are expecting too much from us.

"[The reason] why historically the youth league has not spoken is because the statement presented by the president is the statement of the NEC where the leagues are represented," Mantashe said.

The celebrations come at a time when the ANC is embroiled in a bitter battle between Zuma and Malema's youth league, which wants the president replaced by his deputy, Kgalema Motlanthe, at the party's national conference in December this year.

At "mini-rallies" addressed by Malema this week as part of the build-up to today's festivities, the youth league leader called on his supporters to be on their best behaviour when Zuma speaks. He did, however, warn that hostilities would resume in weeks to come.

The decision to allow only Zuma to speak has angered some ANC leaders who believe that former president Thabo Mbeki, as one of the former party leaders who is still alive, should have been allowed to address the gathering.

Members of the ANC's centenary steering committee were at odds this week over Mbeki's role during the event. The final decision was that he would be limited to the small roles of lighting a torch at midnight and handing it over to Zuma today.

Mantashe said there was no need for Mbeki to speak at the centenary as Zuma, as president of the ANC, would speak on behalf of every ANC member.

"There can be no special dispensation for a former president because there is a sitting president. There are 12 families of all the ANC presidents ... living and dead ... all the families are represented here," said Mantashe.

Zuma's address will capture the ANC's rich history and, according to his aides, reveal a new vision to professionalise the ANC.

Among the heads of state expected to attend the ceremony are Nigerian President Goodluck Jonathan, his Ugandan counterpart President Yoweri Museveni and Equatorial Guinea President Teodoro Obiang Nguema.

It could not be established whether Venezuelan President Hugo Chavez had arrived in South Africa as expected.

A number of heads of state arrived yesterday, among them President Andry Rajoelina of Madagascar, President Armando Guebuza of Mozambique and President Michael Sata of Zambia. Zimbabwe's President Robert Mugabe was too ill to travel and was replaced by Deputy President Joyce Mujuru. The heads of state will be accommodated at the exclusive Woodland Hills Estate in the town.

Preparations for the celebrations were not without hiccups as both Zuma and Motlanthe failed to show up at side events where they were meant to officiate.

Motlanthe's office laid the blame on the ANC's steering committee for failing to inform it that it had billed the deputy president to officially open an ANC golf challenge on Friday.

Motlanthe was on holiday in Cuba and returned on Thursday. His spokesman, Thabo Masebe, said Motlanthe was too tired to travel before the weekend.

An official at the Presidency said the steering committee had not confirmed with Zuma's office his availability to address a rally in Thaba Nchu.