HONG KONG, China (AFP) — One of the most senior Catholic clerics criticized the Vatican's response to the arrest and prosecution of a 90-year-old Hong Kong cardinal for allegedly violating its national security law.

Cardinal Joseph Zen went on trial alongside five fellow democracy supporters on Monday over their role in running a fund to help defend people arrested in anti-government protests.

One of Asia's highest-ranking Catholic clerics, Zen was originally detained earlier this year under the national security law that Beijing imposed on Hong Kong to quell dissent.

"We abandoned him," German Cardinal Gerhard Mueller told the Italian newspaper Il Messaggero earlier this month saying he was disappointed that a recent consistory — a gathering of cardinals in Rome — did not speak in support of Zen.

"This cardinal will be sacrificed on the altar of reason, to defend and implement the diplomatic agreement with Beijing," he added.

The Vatican has been muted on Zen's arrest, saying it was concerned and "following the development of the situation very closely."

Pope Francis declined to comment on Zen's arrest when asked about it earlier this month but said he was determined to continue pursuing a dialogue with Beijing.

Zen's prosecution comes at a sensitive time for the Vatican, which is working to renew its controversial agreement with Beijing later this year over the appointment of bishops in China.

His arrest for "colluding with foreign forces" shocked the city's Catholic community and renewed criticism of the Vatican's warming ties with Beijing, including from fellow senior clerics.

Shanghai-born Zen was critical, calling the deal a "sellout" of China's underground Catholics who have faced persecution for remaining loyal to Rome and refusing to recognize the Communist Party-sanctioned Catholic Patriotic Association.

Not guilty plea

Police have not yet charged Zen with a national security offense, which can carry up to life in jail.

Instead, he and his fellow defendants, including activist and singer Denise Ho and veteran human rights barrister Margaret Ng, are being prosecuted for the less serious offense of failing to properly register their defense fund as a society.

If convicted they face a fine of up to HK$10,000 ($1,274) but no jail time. All have pleaded not guilty.

None of the defendants spoke to reporters on their way into West Kowloon court on Monday morning.

Zen's group acted as trustees of a now-defunct fund that helped pay legal and medical costs for those arrested during huge and sometimes violent democracy protests three years ago.

Zen's "612 Humanitarian Relief Fund" is charged with not properly registering under the Societies Ordinance, a colonial-era law from 1911.

The fund disbanded after national security police demanded it hand over operational details, including information about its donors and beneficiaries.

As proceedings got underway Monday, lawyers for both sides argued over whether the fund needed to register as a society.

The court is not sitting on consecutive days and the trial is expected to conclude in early November.