Twenty people implicated in a deadly attack on the US embassy in the Tunisian capital last year, some of whom could face the death penalty, insisted they were innocent as their trial opened on Tuesday.

Hundreds of angry Islamist protesters attacked the US mission in Tunis on September 14 after an American-made film mocking their religion was published on the Internet.

Four of the assailants were killed and dozens wounded in the violence, which saw protesters storm the embassy and torch a neighbouring American school.

Questioned individually by the judge, the accused, nine of whom have been remanded in custody, denied having taken part in the protest or attacking the embassy and the police.

Defence lawyers strongly criticised the trial and the main charges, including premeditated attacks organised by an armed gang, with sentences ranging from five years in jail to possible death penalties.

No executions have been carried out in Tunisia since 1991 and death sentences are rarely pronounced.

"These protests were part of a spontaneous reaction throughout the (Muslim) world against attacks on our sacred symbols," said one of them, Slah Barakati.

He demanded that charges be dropped, saying that the trial was a result of the Tunisian judiciary bowing to pressure from the West.

"These Tunisians are in court to please the United States and the European Union," Barakati told the judge.

Another lawyer criticised the "confessions extracted by the police under pressure and threats".

The government has accused Saif Allah Bin Hussein, a former Al-Qaeda fighter in Afghanistan known as Abu Iyadh who heads Tunisia's main Salafist movement Ansar al-Sharia, of orchestrating the embassy attack.

Abu Iyadh has been on the run from the police since September, but none of the movement's leaders have been tried for the embassy attack.

Tunisia has been rocked by waves of violence blamed on radical Islamists since the 2011 revolution that overthrew the regime of strongman Zine El Abidine Ben Ali, culminating in the embassy attack.

The coalition government, led by Islamist party Ennahda, was sharply criticised last year for failing to rein in the extremists and prevent the violence.

But in recent months it has taken a much tougher stand, especially since the discovery in April of jihadist groups on the border with Algeria with links to Al-Qaeda.

Earlier this month one person was killed and about 20 wounded in clashes between police and Ansar al-Sharia supporters.