By Stephen Gibbs
BBC News, Havana
Opposition figures in Cuba say at least 20 Cuban dissidents have been detained as part of a crackdown against the country's political opposition.
Martha Beatriz Roque is said to be among those arrested
They are said to include the three main organisers of a public meeting of dissidents which took place in May.
The Cuban government has not confirmed the arrests.
Cuba's long-standing position is that dissidents are not representative of public opinion, but rather mercenaries in the pay of the US.
According to relatives of those being held and Cuban opposition sources, Cuban state security agents visited scores of houses across Havana on Friday morning.
The majority of those dissidents that are currently in custody were intending to take part in a protest outside the French embassy.
Some Cuban dissidents are unhappy at what they see as France's particularly soft line on Cuba.
Among those detained are three of President Castro's most outspoken critics - Martha Beatriz Roque, Felix Bonne and Rene Gomez.
About 200 delegates attended the rare public meeting they helped organise in May. At the meeting, delegates approved a resolution in which they described Cuba's system of government as Stalinist and its economic policies as obsolete.
This clampdown might be linked to that meeting, but it also comes at a time when there are some signs of general discontent in Cuba.
Weeks of extended power cuts, worsened by the after effects of Hurricane Dennis, have tested the patience of many Cubans.