Cuban police have detained, threatened or expelled from Havana some 20 dissidents over the past five days in the opening salvo of a campaign to avert disruptions during the Iberoamerican Summit, human rights activists say.

''I expect most of us will be under house arrest when it happens,'' Havana dissident Ileana Someillan said of the summit of heads of government from Latin America, Spain and Portugal set for Nov. 15-16.

Cuban Foreign Ministry spokesman Alejandro Gonzalez Thursday said that although the visitors will be allowed to meet dissidents, Cuba will ''punish'' any ''counterrevolutionary activities'' aimed at disrupting the summit.

''The leaders will have time and total freedom of movement,'' Gonzalez said. ''Although many may use that time to do things that we don't like, we will not display any signs of offense.''

None of the heads of government attending the summit -- five will boycott it -- have yet announced meetings with dissidents, although Spanish President Jose Maria Aznar is reported to be trying to arrange one.

Opponents of President Fidel Castro hope to hold several activities in coming days, from a dissidents' summit to a peasant conference, to capture international attention for their demands for freedom and democracy.

But Elizardo Sanchez, head of the Cuban Commission on Human Rights and Reconciliation, said Thursday in a telephone interview from Havana that the Castro government appears bent on blocking any public protests.

Sanchez said three dissidents were arrested over the last five days, with two remaining under detention Thursday: Oscar Elias Biscet, 37; and Carlos Rios Otero, an activist in several groups opposed to Castro.

It was the 25th time in the past year that police have detained Biscet, a physician who heads the Lawton Human Rights Foundation, an outspoken group that has staged several small but unprecedented public protests.

His wife, Elsa Morejon, said police told her after Biscet's arrest Wednesday morning that she would be able to visit him in jail next Wednesday -- indicating they plan to hold him for a while.

''The most likely thing is that they will keep him until the summit is over,'' she told The Herald.

Sanchez said some 17 other dissidents were harassed or threatened in an ''arbitrary'' manner, adding that state security agents showed no judicial orders backing their actions.

A few dissidents who were in Havana illegally were deported to their provincial homes, Sanchez said. Cubans require government permits to move permanently from one municipality to another.

And some opposition figures living outside Havana were told by security agents that they would not be allowed to travel to the capital until the summit ends, he added.

Sanchez said he did not give credence to speculation that the government is preparing a makeshift outdoor prison outside Havana to hold all the dissidents expected to be rounded up during the summit.

''This government has a surplus of penal capacity,'' he said.

Word on the crackdown came two days after Castro, in a five-hour television appearance, named some 30 dissidents as ''counterrevolutionaries'' and accused them and U.S. diplomats in Havana of trying to disrupt the summit.

Castro later told journalists that Cuba's best-known dissidents, the so-called Group of Four, would not be allowed out of prison ''one day'' before they serve their sentences, according to published reports.

Several foreign governments have asked Castro to free Marta Beatriz Roque, Vladimiro Roca, Rene Gomez Manzano and Felix Bonne, convicted of sedition in a closed-door trial earlier this year and sentenced to prison terms ranging from 3 to 4 1/2 years.