The Cuban government has begun releasing jailed dissidents in a political concession brokered by the Catholic church that could ease the way towards a gradual thawing of relations with the US.

Raúl Castro's regime agreed last week to free 52 activists once viewed as enemies of Fidel Castro's revolution – almost a third of the 167 inmates classified as political prisoners by the independent Cuban Commission for Human Rights and National Reconciliation. At least 17 are set to go to Spain to live in exile.

The families of several prisoners were tipped off on Saturday that their relatives had been released. The wives of dissidents Pablo Pacheco and Luis Milan were phoned by fellow inmates. Barbara Rojo, the wife of Omar Ruiz, told the Associated Press: "They [Cuban officials] called me to tell me to get ready to leave, because they would be around to get us."

A fourth prisoner, José Paneque, called his family to tell them he was being moved to Havana from a jail in Las Tunas, an eastern province, according to AFP.

Meanwhile, there was a reminder that the old guard endures, as pictures emerged of Fidel Castro out in public for the first time in four years. Looking frail but animated in a white Nike tracksuit, the ailing 83-year-old retired dictator was snapped smiling and chatting with staff during a visit to the National Centre for Scientific Investigation in Havana.

An officially sanctioned website, Cubadebate, attributed the pictures to his son, Alex. Castro fell ill in 2006 and underwent stomach surgery. After 49 years in power, he handed over office to his brother, Raúl, two years ago. Since then, he has virtually disappeared from public view. Rosa Baez, a journalist for the state-sanctioned media, commented: "He is thin but he looks good, and according to our director, he is very good mentally."

Since Fidel Castro's retirement, Cuba has shown tentative signs of reform – bans on ownership of mobile phones and computers, and on Cubans staying in tourist hotels, have been relaxed.

In Washington, President Barack Obama has eased restrictions on Cuban exiles travelling from the US to Cuba, although he criticised the regime for continuing to treat its citizens with a "clenched fist".

The release of Cuban dissidents, many of whom were rounded up in an anti-democratic crackdown in 2003, follows international condemnation over the death of hunger striker Orlando Zapata Tamayo in February, after refusing food and drink for 85 days in protest at his imprisonment. Wives and relatives of jailed dissidents have been staging weekly marches in Havana to keep up pressure on the government.

Moves towards the mass release began when the Spanish foreign minister, Miguel Ángel Moratinos, brokered a meeting between Raúl Castro and Cuba's Catholic cardinal, Jaime Ortega, last week. The Vatican expressed satisfaction at signs of progress secured by the church.

"The world looks with hope at the events that are coming out of Cuba," a Vatican spokesman, Federico Lombardi, told Vatican Radio on Saturday. "We all hope that this path continues."

The US secretary of state, Hillary Clinton, said she was "encouraged" by the agreement to release dissidents. "We think that's a positive sign. It's something that is overdue, but nonetheless, very welcome."

In Congress, many Democrats favour easing America's decades-long embargo on Cuba. The House agriculture committee voted two weeks ago to reverse a ban on American citizens visiting the island. But legislation stands little chance of enactment without support from the White House.