* Movement turning into grass-roots rebellion
* Situation in mining sector unacceptable - Zuma
* Investors worried about risks to economy
* Amplats workers demanding huge pay hike
By Pascal Fletcher
RUSTENBURG, South Africa, Sept 13 (Reuters) - Striking South African platinum miners delivered much higher pay demands on Thursday and threatened to spread industrial action further, deepening a crisis that is becoming the biggest threat to the ruling ANC since the end of apartheid.
In the face of the spiralling labour unrest in Africa's biggest economy, President Jacob Zuma said the government would crack down on anybody stirring up trouble.
"It is not just the miners striking. It also some people of some description who are going there to instigate miners," said Zuma, who faces an ANC leadership election in December. "We are going to be acting very soon," he told parliament in Cape Town.
What began as an industrial dispute has evolved into a grass-roots rebellion by blacks who have seen little improvement in their lives since white minority rule ended 18 years ago.
At a soccer stadium in the heart of the platinum belt, thousands of workers heard a call from one protest leader for a national strike to "bring the mining companies to their knees".
"On Sunday, we are starting with a general strike here in Rustenburg," Mametlwe Sebei, from a fringe Marxist political party, told the workers near the town which lies 100 km (60 miles) northwest of Johannesburg.
He drew mild applause from the crowd, armed with sticks and machetes, and it was unclear if the strike call would be heeded.
Despite the weapons, the strikers insisted their push for a sharp hike in wages was peaceful - even after the Aug. 16 police shooting of 34 protesters at Lonmin's nearby Marikana platinum mine.
"There should be no blood," one placard read.
As investors started to fret about the impact on wider economic growth, the rand fell more than one percent against the dollar - compounding a 3 percent slide on Wednesday. Mining accounts for 6 percent of South Africa's output.
Most men at the soccer stadium said they worked for top producer Anglo American Platinum, commonly known as Amplats, which suspended operations at its four Rustenburg mines on Wednesday after they were blockaded by marchers.
A group of more than 100 chanting strikers, many waving sticks and "knobkerry" clubs, accompanied protest leaders as they delivered a written memorandum laying out their demands to Amplats management offices near the Bleskop stadium.
Police armoured vehicles kept the larger crowd of miners inside the stadium, within sight of a white clubhouse painted with Amplats corporate slogans such as "We value and care about each other" and "We are one team".
The demands were for an increase of basic pay and allowances to 16,000 rand ($1,900) a month - nearly three times their current salary and more than double per capita GDP in the continent's richest country.
As the stick-waving miners accompanied their leaders back to the stadium, they chanted: "We won't give up!"
They said they would not return to work until top management - including Cynthia Carroll, chief executive of Amplats parent company Anglo American - came to hear them out.
"She must come to the workers," a 32-year-old called Kasigo told Reuters. "If they don't come, we won't work."
Amplats confirmed it had received the demands and was monitoring the situation closely.
The labour unrest began with a violent six-week strike at Impala Platinum in January. It intensified in mid-August, sending platinum prices up 20 percent since then.
It stems from a challenge by the small but militant Association of Mineworkers and Construction Union (AMCU) to the dominance of the ANC-affiliated National Union of Mineworkers(NUM) and is also spreading into the gold sector.
World number four producer Gold Fields said NUM officials came under attack when they tried to address wildcat strikers at its KDC West mine near Johannesburg, where 15,000 men downed tools last week.
ANC renegade Julius Malema - the de facto face of an unofficial "Anyone but Zuma" rebellion in the ANC - has entered the fray, accusing the polygamous Zuma of being more interested in arranging weddings than trying to clean up the mess.
Ministers and NUM leaders have dismissed Malema as an irresponsible opportunist but the expelled Youth League leader is becoming a star for the legions of South Africa's impoverished black majority.
"People who believe that Malema does not present a danger to South Africa have missed the point," said Richard Faber, a fixed income trader at Johannesburg brokerage Worldwide Capital. "It is his ideology that presents the danger and that is gathering momentum."
The platinum price held steady on Thursday near the 5-month high it hit following the Amplats shut-downs.
Amplats shares fell as much 1.8 percent in early trade before bouncing to be up 1.3 percent by 1400 GMT.