CARACAS, Venezuela (CNN) -- More than 20 people were killed after violence broke out at a Venezuelan prison over the weekend, an official said Monday.

Two "heavily armed" groups inside the Yare I prison clashed Sunday, Iris Varela, the South American nation's prison minister, told state-run VTV.

An inmate's family member was among the dead, she said.

Anxious relatives waited outside the prison Monday, demanding that authorities release the names of those who were killed or wounded in the fighting, CNN affiliate Globovision reported.

More than 300 prisoners died in Venezuelan prisons in the first half of 2012, according to the nonprofit Venezuelan Prison Observatory. Inmates' family members have accused officials of abusing prisoners. Authorities have blamed prisoners for the violence.

There are about 47,000 inmates in Venezuela's 33 prisons, which were originally built to house 12,000 prisoners, the observatory says.

Varela told VTV on Monday morning that the prison was calm and that authorities would punish those responsible.

"They cannot keep committing crimes in the prisons and go unpunished," she said.

The Yare prison is in the northern state of Miranda, where at least 25 people died after a standoff at another prison last year.

Prison violence has become a high-profile issue on the campaign trail leading up to Venezuela's October 7 presidential vote.

Opposition candidate Henrique Capriles Radonski and his supporters have proposed prison reforms, criticizing officials for delayed court proceedings and substandard facilities.

"This government failed and was defeated by violence inside and outside prisons. The proof of it is in the homicide statistics of recent months," Leopoldo Lopez, Capriles' campaign manager, said in May.

On Monday, Varela said the opposition has no moral standing to criticize the government's handling of the situation, arguing that such problems have also persisted in states led by opposition governors.

"They cannot attack us for a problem that we know and recognize exists," she said, according to VTV.

Journalist Osmary Hernandez contributed to this report.