A Bangladesh photographer aligned with the country's main opposition party has been jailed for life over a murder more than a quarter century ago, a verdict his lawyer says is politically motivated.

Nooruddin Ahmed, a long-time photographer for Bangladesh's opposition leader Khaleda Zia, was among 27 people handed life sentences on Tuesday by a tribunal ruling in the murder.

Prosecutors said the accused were all members of the Freedom Party, a political outfit that was accused of shooting dead a young Bangladeshi in 1990 in the northern district of Mymensingh.

Ahmed was among those arrested and charged over the murder but the case languished for years before it was transferred to a speedier tribunal.

"All 27 accused, including Nooruddin Ahmed, were found guilty under section 149 of the Bangladesh Penal Code and were jailed for life", prosecutor Syed Shamsul Haque said.

However, Sanaullah Miah, Ahmed's lawyer, said his client was never a member of the organisation, and was covering an event as an official photographer at the time of the murder.

"He was convicted because he is the photographer of former Prime Minister and BNP [Bangladesh Nationalist Party] leader Khaleda Zia," he said, adding that Ahmed has been Zia's personal photographer since 1991.

The case was "politically motivated" due to Ahmed's associations with Bangladesh's opposition and the prosecution lacked witnesses, Sanaullah said.

The Freedom Party was founded by retired army officers and has been linked to bombings and other violence.

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The group rose to notoriety after several of its members were convicted and hanged over the 1975 assassination of Bangladesh's founding leader Sheikh Mujibur Rahman, father of the current leader Sheikh Hasina.

Ahmed's brother Mosaddek Ali Falu, a long-time political secretary of Zia, was arrested in 2015 for arson at the height of anti-government protests that gripped Bangladesh for months.

BNP officials have said that tens of thousands of their activists and supporters have been arrested and prosecuted by the government since 2014, when the party boycotted a controversial general election over fears it would be rigged.

Zia herself faces at least 30 cases of alleged corruption and violence, her lawyers said.