* Plumes of black smoke billow across the capital
* Biggest display of public anger in over a year
* Similar protests broke out last year but fizzled out
* Internet access cut off (Adds more details about the protests)
By Khalid Abdelaziz and Ulf Laessing
KHARTOUM, Sept 25 (Reuters) - Sudanese protesters torched cars and petrol stations and threw rocks at police in the capital Khartoum on Wednesday on a third day of protests against a cut in fuel subsidies.
Plumes of black smoke sprang up around the horizon as security forces fired teargas to disperse the biggest display of public anger against President Omar Hassan al-Bashir's government in over a year.
Internet access was cut off all over Sudan after activists began sharing images of the unrest on social media, although the cause of the outage was not immediately clear.
Bashir has avoided the mass demonstrations that unseated rulers in Tunisia, Egypt and Yemen and led to civil wars in Libya and Syria, but discontent has been simmering against corruption and soaring prices.
"The people want the fall of the regime!" protesters chanted, echoing the trademark chant of the "Arab Spring" demonstrations. "No, no to high prices!"
Mobs set fire to a university building and several petrol stations in Khartoum. They blocked a main road to the airport near the luxury Rotana hotel, used by diplomats and businessmen, and torched several cars in the parking lot, witnesses said.
A Reuters reporter saw police fire volleys of teargas grenades into the crowd, while hundreds of officers and plainclothes security agents armed with guns or batons rushed to the city centre. Others were sitting on the roof of government buildings. Agents detained some 20 protesters, who were driven away in pickup trucks.
There was no immediate comment from the authorities on the unrest.
Many of Wednesday's protests around the capital mustered only a few dozen or at most a few hundred people and dispersed after a short time, making it hard to get an exact idea of how many had joined. But the total number in Khartoum was likely to have been in the thousands, and there were also demonstrations in the Red Sea coastal city of Port Sudan.
Similar protests broke out in June last year after the government reduced fuel subsidies as part of a plan to contain its ballooning deficit, but they ended in the face of a security crackdown and Sudan's intense summer heat.
Bashir said on Sunday night that remaining subsidies would be lifted, but did not give details or a timeline. The next morning, prices of petrol, gasoline and cooking gas nearly doubled.
The roots of Sudan's economic crisis lie in the secession of South Sudan in July 2011. The new nation took about three-quarters of Sudan's oil output, the lifeblood of its economy.
Crude exports were the government's main source of income and of the foreign currency it needs to import food and other goods for its 32 million people. Inflation soared and the pound lost over half its value against the dollar on the black market.
On Wednesday, the dollar bought about 8.2 Sudanese pounds on the black market, compared to about 7.3 pounds last week before the government announced it would cut fuel subsidies.
Khartoum had hoped to maintain some subsidies by boosting gold exports to replace oil revenues, but has been undermined by a recent fall in global gold prices.
The government says annual inflation eased to 23.8 percent in July from 37.1 percent in May, but independent analysts put the actual rate at 50 percent or even higher.
There was no word on casualties in Wednesday's unrest. Two people were killed during protests in the Khartoum area on Tuesday, relatives who named the victims told Reuters.
Police have confirmed only one death that day, saying a robber had killed an unnamed man. Activists blamed government forces for the man's death.
It remains to be seen whether the most recent round of protests will gather momentum or fizzle out like previous bouts of unrest in the last two years.
Bashir, who came to power in a coup in 1989, has weathered multiple armed insurgencies, years of crippling U.S. trade sanctions and a warrant for his arrest from the International Criminal Court.
Sudan's opposition parties, run by older men, are weak, divided and have little appeal for young people demanding drastic improvements in living standards and political change. (Reporting by Ulf Laessing and Khalid Abdelaziz; Writing by Alexander Dziadosz in Beirut; Editing by Kevin Liffey)