Helen Zille: South Africa's vigorous voice of opposition

(CNN) -- Tough and charismatic, South African opposition leader Helen Zille is one the country's most powerful women.

Since she was elected leader of the Democratic Alliance in 2007, Zille has been concentrating her efforts on broadening the appeal of her party.

In order to challenge a longstanding political powerhouse such as the ruling African National Congress (ANC) -- which took about 66% of the vote in the last general election in 2009 -- Zille wants to move her party away from just being seen as the political home of South Africa's white liberals.

The Democratic Alliance (DA), which has a strong support base among white people and South Africans of mixed race, won 16% of the national vote two years ago.

"Growing the base of a political party on the foundation of a political philosophy and a value set is very difficult in a racially-divided context, where people tend to see politics as a choice between different race groups," she says.

"And so growing your base across all barriers requires a mix of strategies," adds Zille, who is also premier of the Western Cape province, where her party beat the ANC.

Under Zille's leadership, the DA has become the most vocal opponent of the ANC, which is led by President Jacob Zuma.

Before the 2009 election, the party strongly opposed Zuma's presidential bid, with Zille spearheading the DA's legal campaign against the dropping of his corruption case.

An investigative journalist before entering the political arena, Zille made her name in 1977 when she exposed how activist Steve Biko was killed by apartheid police while in detention.

Her story in the liberal Rand Daily Mail refuted the regime's version that Biko's death was the result of a hunger strike and was seen as a watershed moment for South Africa as it uncovered one of the darkest episodes in the country's history.

"I have no doubt that our politics would have taken a very different turn had he lived and had he led and when he died in detention we all knew what a critical moment that was and we all understood that a big cover-up was in progress," Zille says.

Soon after breaking the Biko story, Zille left journalism to become a political activist. She worked in a number of non-governmental organizations and human rights groups, including the Black Sash, a women's civil rights movement.

She later joined the DA and became mayor of Cape Town in 2006 -- marking the first loss for the ANC in a major South African city. In May 2007, Zille was elected party leader of the DA and in 2009 she became premier of the Western Cape province.

While Zille admits that winning elections in a big city like Cape Town and a major province like Western Cape is a big improvement for the DA, she is determined to make further gains.

"We've got to build this democracy. And democracies, especially on our continent, have failed because too much power has been concentrated in too few hands, been abused and led straight to corruption and the criminal state. We're not going to have that in South Africa."

Her vigorous stance against Zuma has not been without controversy. Zille and ANC leaders have often traded bitter words, and she's been lampooned by national media. But Zille says she remains unfazed by vehement criticism.

If anything, she says, such attacks indicate that the DA is becoming a risk to the ANC. And with the local government elections scheduled for later this year, Zille says she's preparing to deal with the harshest criticism she's ever faced.

"This election campaign that we're going into now for 2011 is going to be vicious beyond anything that you can ever imagine and I'm already steeling myself to face the kind of attacks that I've never faced before," she says.