At least one person was killed and eight were wounded, including three policemen, in running clashes between police and Zulu protesters in Durban in KwaZulu-Natal province on Saturday.
President Nelson Mandela expressed his "extreme concern" at the incident and said if the situation worsened, extra security forces could be sent to the province.
The violence came as negotiators were staging a last-ditch attempt to break a deadlock which could stall implementation of the country's final post-apartheid constitution.
Presidential spokesman, Joel Netshitenzhe, said the President "has expressed extreme concern about what happened in Durban yesterday. The situation is being reviewed on a continual basis and if it worsens, security forces will be sent to KwaZulu-Natal."
The cabinet has to decide whether local elections on 29 May, already delayed twice, should go ahead or be postponed again.
The African National Congress says alleged voter registration fraud and political killings and intimidation mean a fair election is impossible.
The ANC's main rival, Chief Mangosuthu Buthelezi's Inkatha Freedom Party, says the ANC is just scared of losing the election in KwaZulu-Natal, where more than 14,000 people have been killed since the mid-1980s in a bloody turf war between the two parties.
Police said the province was calm but tense yesterday. "The signs are there for conflict. Since the electioneering started there has been tension," provincial police spokesman Bala Naidoo said.
Mr Buthelezi, who is also interior minister in Mr Mandela's coalition cabinet, said the elections must go ahead.
"I make this appeal to the President: for the sake of democracy ... let the 29 May elections go ahead as planned," he told supporters at an election rally.
The Zulu leader, whose party has boycotted the constitution-writing process since last year over demands for international mediation, launched a blistering attack on the draft constitution.
"Let no-one be in any doubt that this new constitution is the gravest threat to our liberty in existence.
"This new constitution is nothing less than a recipe for a one-party state," Mr Buthelezi said.