European governments fear often overcrowded prisons are virus timebombs during this pandemic and are looking at early releases and other ways to reduce the risk for those behind bars.
The issue of those locked up while whole countries were locked down was discussed during a video conference of EU justice ministers held on Monday.
EU justice commissioner Didier Reynders, who also took part, tweeted that the impact of COVID-19 was looked at, along with "measures taken relating to the functioning of (the) justice system".
Croatia, which chaired the meeting in its role holding the rotating EU presidency, said there was an exchange of information on prison and pre-trial detention in relation to the pandemic.
While no EU-wide policy has yet emerged, individual member states have already taken steps to reduce penitentiary populations, aware that social distancing is impossible for those incarcerated.
France, for instance, has since mid-March reduced its prisoner and detainee population by nearly 10 percent, by putting off custodial sentences for less-serious crimes, suspending terms for medical reasons, freeing some who were jailed awaiting trial, and allowing early release.
That has cut its number of inmates to 66,300, the French justice minister told AFP last week -- though many of its 188 prisons are overcrowded.
- Alternative to prisons 'imperative' -
Greece has taken similar measures to release 1,500 prisoners.
Non-EU member Britain, too, has announced the early release of 4,000 prisoners with less than two months left on their sentences, barring those convicted of violent, sexual or paedophilia crimes or posing national security threats.
The human rights commissioner for the Council of Europe, an organisation promoting rights and democracy whose membership goes well beyond the EU, called on Monday for more countries to follow suit.
"The resort to alternatives to deprivation of liberty is imperative in situations of overcrowding and even more so in cases of emergency," Dunja Mijatovic wrote in a statement.
At the same time, prisons in some countries have cut visiting rights for inmates' lawyers and banned those for family members, sparking episodes of unrest in Italy and in Sweden.
In Romania, three prisoners died and another two were wounded in a riot that broke out in a prison in the northern town of Satu Mare, reportedly because of curtailed visiting hours.
While confirmed cases of infection are currently relatively low, concerns are swiftly intensifying that the virus could take hold within prisons, where convicts share cells, showers and eating halls.
Prison staff also complain they lack sufficient protective gear to safely carry out their duties.
- Death toll mounting -
Already in Italy, a 76-year-old inmate and two guards have died of COVID-19. Another 19 of the country's 58,000 prisoners have tested positive, along with 116 guards, according to official figures given last week.
In Spain, one female prisoner and a guard have died. Another six prisoners are infected, its interior ministry said.
France also has recorded the deaths of one prisoner and one guard, along with another 48 prisoners infected as well as 114 guards.
In Britain, two guards at London's Pentonville prison died last week after coming down with COVID-19 symptoms, their union said.
Belgium has reported that 32 of its guards have tested positive, as have four inmates.
Those figures underline the growing risk in penitentiaries.
In Russia, Eva Merkatcheva, a member of a prisoners' rights group known as the Commission of Observers, told AFP that "the situation in Russian prisons could quickly become a catastrophe, especially in holding centres where detainees are squeezed together in sometimes overcrowded cells".
As a stopgap measure to try to prevent prison revolts in this time of crisis, authorities in certain countries have eased some conditions inside.
France has rolled out free television, telephone credits and assistance for those in need. Spain has handed out smartphones to allow prisoners to make video calls.