The two attackers who died, named as Yassine Laabidi and Hatem Khachnaoui, were killed in a gunfight with security forces inside the building.

Hundreds of people gathered for a mass in the cathedral in Tunis on Saturday, lighting candles to remember the victims, who also included three Tunisians, in a ceremony attended by government ministers.

Outside, there was a heavy police presence along the central Habib Bourguiba boulevard. But Tunis was calm, with a music festival going ahead in the city centre.

Wednesday's assault, the deadliest involving foreigners in Tunisia since a 2002 suicide bombing on the island of Djerba, came at a fragile moment for a country just emerging to full democracy after a popular uprising four years ago.

The government said the two gunmen had trained in jihadi camps in Libya before the attack at the Bardo museum inside the heavily secured Tunisian parliament compound. Japanese, French, Polish, Italian and Colombian visitors - as well as one Briton, Sally Adey, 57 -  were among the victims.

Authorities have arrested more than 20 suspected militants, including 10 believed to be directly involved in the Bardo attack, Interior Ministry spokesman Mohamed Ali Aroui said.

"There is a large-scale campaign against the extremists," he said. The ministry released a photograph of another suspect and asked Tunisians to help with information.

The government plans to deploy the army to major cities to improve security following the shootings.

Islamic State militants have claimed responsibility for the attack, but social media accounts tied to an al Qaeda-affiliated group in Tunisia have also published purported details of the operation.

Whoever was responsible, the Bardo attack illustrates how Islamist militants are turning their attention to North Africa. A particular focus is neighbouring Libya, where two rival governments are battling for control, allowing Islamic State to gain a foothold.

The United States is increasingly worried about the growing presence of Islamic State militants in Libya.

In an interview with Paris Match magazine, Tunisian President Beji Caid Essebsi said there were as many as 10,000 young Tunisian jihadists in all.

"Among the often desperate young unemployed, the call to jihadism has worked," he said.

"Four thousand Tunisians have joined jihad, in Syria, Libya and elsewhere, and some 500 have already come back here, where they pose a threat. That is not to mention the five or six thousand others we have succeeded in preventing from leaving."

U.S. officials said that because of its strategic position, Libya has become a springboard for would-be fighters from across North Africa wanting to link up with Islamic State.

The militant group controls large swathes of Iraq and Syria, and claimed responsibility for suicide bombings at two mosques that killed at least 137 people in the capital of Yemen on Friday.

Four years after a popular revolt toppled autocrat Zine El-Abidine Ben Ali, Tunisia has completed its transition to democracy with free elections, a new constitution and compromise politics between secular and Islamist parties.

But the attack threatens to hit the economy of a country that is heavily reliant on drawing foreign visitors to its beach resorts and desert treks. Authorities have tightened security at hotels and tourist spots.

Additional reporting by Associated Press.