South Africa's main opposition, the Democratic Alliance, on Sunday claimed activists of the ruling ANC party stoned two buses full of its supporters on their way to an election rally.
One of the buses was attacked near the FNB stadium, the venue of the 2010 World Cup final near the Soweto township of Johannesburg, where the African National Congress party is holding its final campaign rally before elections next week.
Police said the driver had been taken to hospital for treatment after the attack.
Another bus was targeted as it drove through the township of Alexandra on the way to a DA election campaign concert in Soweto's Walter Sisulu Square, the party said in a statement.
"Two severe stoning attacks have so far occurred in Alexandra and the FNB stadium vicinity," the statement said.
South Africans go to the polls on Wednesday in what is expected to be the biggest test of the ANC since it came to power 20 years ago.
Violent demonstrations against poor public services have broken out in several townships ahead of the polls, but there have been no serious clashes between party supporters.
Police confirmed two buses were pelted with rocks on Sunday morning in Soweto and Alexandra, but would not say who was behind the attack.
"At this stage I can confirm that there was one bus stoned in the Soweto area, where a driver was injured," police spokesman Neville Malila told AFP.
"Apparently the bus was carrying DA supporters, but we don't know who stoned the bus," he said.
"In Alexandra, we also received the report of a bus that was stoned and no injuries were reported," said Malila. No one has been arrested for the attack.
Spokesmen for the ANC could not be reached for comment.