In London, people waited up to two hours to cast their vote, with many expressing disapproval of the African National Congress (ANC), the former liberation movement that has run South Africa since the end of apartheid in 1994.
Opposition parties such as the Congress of the People (COPE), an ANC splinter group formed in December, are likely to fare well among the tiny number of educated expatriate voters.
However, the ANC, which still commands huge respect among ordinary blacks for its long fight against white minority rule, is likely to win the two-thirds parliamentary majority that will allow it to change the constitution at will.
"I’ll probably vote for COPE. They just seem to be the strongest opposition. People are fed up with the way the government is running things," IT worker Graham Warnasuriya, 30, told Reuters outside the ornate South African embassy building.
Others praised senior ANC figures such as finance minister Trevor Manuel but said their respect was tempered by the prospect of populist party leader Jacob Zuma becoming president of Africa’s biggest economy after the April 22 vote.
"Unfortunately if you vote for Manuel, you get Zuma. It’s like having a marriage proposal with Pamela Anderson, and when you wake up, you get Frankenstein," said 32-year-old shipping industry worker Claudio Christie.
The line of voters snaking 200 metres (yards) by London’s Trafalgar Square was only allowed by a court ruling in March giving overseas South Africans the right to vote for the first time since 1994.
This month’s decision by prosecutors to drop graft charges against Zuma on a technicality gave added ammunition to critics who say the ANC has a dangerous grip on nearly every aspect of public life, including the civil service and judiciary.
On Wednesday, COPE presidential candidate Mvume Dandala, a Methodist bishop, added his voice to those fearful of a slide towards the one-party political dominance that has blighted other African countries.
"Those parties that have been leading the liberation struggle have very often become the sole parties that lead the people, without any of the meaningful opposition that makes a democracy," he told a news conference in Johannesburg.
"In most instances, they virtually turn into dictators," he said. "We are beginning to see something of that here in South Africa."
Zuma has maintained his innocence and said the charges were a political plot to prevent him becoming president.
COPE was formed by disgruntled ANC members in December after the party pushed Zuma rival Thabo Mbeki out of the presidency.
Analysts said at the time COPE might eat into the ANC’s dominance given public anger over corruption, poor services, poverty and crime — and a looming recession. However, polls suggest there is little sign of it losing its hold.
Dandala declined to say how many seats COPE wanted to win, indicating only that it saw itself coming in second.
"I expect us to do well, and I expect us to do better than any other opposition parties," he said.