(Corrects day of the week in paragraph 1)

HONG KONG (Reuters) -Mitigation pleas for Hong Kong's biggest ever national security trial against the city's democratic opposition kicked off on Tuesday, in what is expected to be the final stage before sentencing that could see some defendants jailed for life.

The 47 democrats were among those arrested in early 2021 for taking part in an unofficial, non-binding poll to select candidates for a legislative election. They were charged with "conspiracy to commit subversion" under a China-imposed national security law.

The marathon case has drawn criticism from countries including Britain, the U.S. and Australia, which say the national security laws have been used to stifle dissent and curb freedoms guaranteed when the financial hub was handed back to China in 1997.

Chinese and Hong Kong authorities say the national security law is necessary and has brought stability.

Most of the democrats have been denied bail and remanded in custody for more than three years in what critics say has been a departure from common law traditions.

The mitigation proceedings come around one month after 14 democrats were found guilty with two acquittals. Several others had earlier pleaded guilty.

The 45 convicted democrats will make mitigation pleas in six batches over the next six weeks, before the sentencing that could range from three years for active participants in the conspiracy, to life for "principal offenders".

On Monday, former law professor Benny Tai, 59, was among the first five defendants to make a mitigation plea.

Tai, who earlier pleaded guilty, was deemed by the judges in the written verdict to be the "brains and primary promoter" behind the primary election to seriously interfere, disrupt or undermine the performance of the government.

Others include those accused of organising the primary election and who have since pleaded guilty and become witnesses for the prosecution - Au Nok-hin, Andrew Chiu and Ben Chung. They were separated from Tai by eight uniformed prison wardens in the dock.

Australian-Hong Kong dual citizen Gordon Ng, 45, is the first among those who pleaded not guilty due to make his mitigation plea.

(Reporting by Jessie Pang and James Pomfret; Editing by Lincoln Feast)

By Jessie Pang and James Pomfret