The spreading of the coronavirus in India's notoriously crowded prisons has prompted authorities to impose jail lockdowns and release thousands of pretrial detainees on parole, as health experts worry that the cramped facilities are serving as breeding grounds that will enable the virus to spread.

Although there are no official numbers on how many inmates have been infected by the virus countrywide, India's correction facilities are slowly recording more and more coronavirus cases, leading authorities to temporarily ban relatives and friends from visiting prisoners.

On Thursday, authorities locked down Nagpur Central Jail in coastal Maharashtra, among the Indian states worst-hit by the pandemic. It was the eighth prison in Maharashtra to be locked down. The move came after 19 inmates in Indore Central Jail in central Madhya Pradesh state tested positive for the virus on Tuesday. Around 250 others who came in contact with them were shifted to a temporary jail.

``It is a terrifying situation. If measures aren't taken soon, then things can become extremely difficult,'' said Madhurima Dhanuka, head of the Prison Reforms Programme for the Commonwealth Human Rights Initiative.

Considering its population of 1.3 billion, India has done relatively well in containing the coronavirus, confirming around 37,000 infections, including 1,223 deaths. On Friday, the government extended the lockdown it had announced in late March for another two weeks, but eased restrictions in some low-risk areas and is now trying to gradually reopen some industries, including agriculture and manufacturing.

Health experts, however, fear that crowded facilities such as prisons can prove deadly, threatening the lives of detainees and guards, as well as the outside population.

The virus has spread rapidly in overcrowded prisons across the world, leading governments to release inmates en masse. United Nations experts and the World Health Organization have urged governments to reduce their prison populations during the pandemic.

In March, India's top court said that it was ``difficult for prisoners to maintain social distancing'' and ordered that detainees convicted of crimes with jail terms of no more than seven years be given parole. Many states started releasing thousands of inmates.

Attempts to reduce the prison population, however, were not enough, critics say.

Indian prisons are highly overcrowded. According to the latest data, put out by the National Crimes Record Bureau in 2018, India has some 450,000 prisoners, exceeding the country's official prison capacity by about 17%. Prisons in New Delhi and neighboring Uttar Pradesh state have the highest occupancy rates, at over 50% above capacity.

Making matters worse, ``the health facilities in prisons are not up to the mark,'' said Dhanuka.

The official data shows that only 4% of total prison expenditure was spent on inmates' medical needs in 2018. It also shows a 40% shortage of medical personnel in Indian prisons.

Dhanuka said the combination of a low health care budget, a shortage of doctors and ``horrible hygiene facilities'' has created ideal conditions for the coronavirus to spread in Indian prisons. ``There is an urgent need to decongest them,'' she said.

While the government said it already has released thousands of prisoners and plans to set more free, worried families whose loved ones are still in jail are distraught. In April, the family of Mian Qayoom, a 73-year-old human rights lawyer from disputed Kashmir who is jailed in New Delhi's Tihar jail, wrote to authorities to urge them to release him on parole due to his ailing health. The family said that Qayoom was a diabetic and had a serious heart condition.

But on April 29, the Indian government said that Qayoom's potential release was not covered in the guidelines issued by the country's top court.

Qayoom was arrested in August under the Public Safety Act, which allows people to be held for up to two years without trial.

``He is most prone to this virus attack,'' said Qayoom's nephew, Mian Muzaffar, who is also his lawyer. ``It's a death sentence to keep him inside India's most notorious and one of the most crowded jails in these times.``