GENEVA — A thriving underground social media network is challenging the Communist government's grip on power and information in Cuba and beginning to bring change, a leading dissident said on Thursday.

But blogger Yoani Sanchez, who has been able to travel abroad due to an easing of exit restrictions, said authorities were still trying to silence critics through detentions.

"There is a network of clandestine information, the volume, speed and efficiency of which you cannot imagine. It is not in megabytes or gigabytes but in terrabytes," Sanchez told a news conference in Geneva.

"One family has an illegal parabolic antenna hidden in a pseudo watertank and can transmit to 200 to 300 families who pay a monthly fee," said Sanchez, who has pioneered the use of social media to challenge Cuba's one-party system.

Those discovered with illegal antennas face confiscation of their equipment and television and a hefty $1,500 fine in a country where the average monthly wage is about $20, she said.

President Raul Castro, who has introduced some economic and other reforms since taking over from his brother Fidel Castro in 2008, is aging and a transition is evitable, Sanchez said.

"There has been a growth of critical civil society, a new phenomenon, through audiovisuals, culture, journalists, bloggers and Twitter. It is no secret that the subsidies from Venezuela that have prolonged the Cuban system will also be phased out.

"All this will accelerate change," she said.

Cuban security forces still repress dissidents but tactics have shifted from imposing long prison sentences to harassment and frequent short-term detention, she said.

"Quite often, dissidents, bloggers, journalists are taking a walk and a private car stops, they are insulted and dragged into a car with three men. They are hit, threatened and dropped of later on the road," she said.

Sanchez said that she had been detained four times since 2009, most recently in November when she was held nine hours and underwent "brutal psychological pressure" from interrogators.

Another leading dissident, Elizardo Sanchez, said on Wednesday the government's easing of travel restrictions had not led to greater freedoms. He was speaking on the sidelines of a U.N. review of Cuba's record.

Cuban officials in Geneva said Yoani Sanchez and Elizardo Sanchez - who are not related - were acting as agents of the United States to bring down the government.