Nineteen people, including 17 foreign tourists, have been killed in a gun attack on the Bardo museum in the Tunisian capital, Tunis, the PM says.
Those killed included citizens from Japan, Italy, Colombia, Australia, France, Poland and Spain, PM Habib Essid said.
Two Tunisians, one a police officer, were also killed, he said.
Security forces killed two gunmen and were searching the surrounding area for accomplices, Mr Essid added.
At the time of the attack, deputies in the neighbouring parliamentary building were discussing anti-terrorism legislation. Parliament was evacuated.
Following the attack, Mr Essid said: "It is a critical moment in our history, and a defining moment for our future.
"We have not established the identity of the two terrorists... Reports are not final, these two terrorists could have been assisted by two or three other operatives."
Security operations were "still under way", he added.
Who are the victims?
According to Prime Minister Essid, 19 people were killed, although some of the countries involved have different totals:
- Two Tunisians, including one police officer involved in the security operation
- Five Japanese
- Four Italians
- Two Colombians
- One national each from Australia, France, Spain and Poland
- Two other victims who were not immediately identified
Earlier reports said that a total of 20 tourists had died, with at least 22 tourists and two Tunisians injured. Other reports suggest up to 50 could have been hurt.
Italian, Polish, South African, French and Japanese tourists were among the injured, Mosaique FM radio reported.
Parliament held an extraordinary session on Wednesday evening.
Tunisian President Beji Caid Essebsi said: "We are in a war against terrorism... we will fight them without mercy."
Analysis: BBC Security Correspondent Frank Gardner
Sadly, this attack did not come out of the blue.
While Tunisia has been spared the catastrophic levels of violence that have plagued other Arab Spring countries like Syria, Yemen and Libya, the country has still suffered from occasional but deadly attacks carried out by Islamist extremists.
In 2013, 22 people were killed. This included a suicide bomber who attacked a beach resort in Sousse. Last year 45 people were killed and already this year the death toll has reached 23, with Wednesday's museum raid following an attack on a mountain checkpoint in February that killed 4 police officers.
In all cases the perpetrators are believed to be jihadists. Tunisian fighters make up a disproportionately high number of foreign recruits to Islamic State (IS) in Syria. More than 3,000 have joined, earning themselves a reputation for ferocity both on and off the battlefield.
At the scene: BBC's Naveena Kottoor in Tunis
The last group of foreign tourists left the Bardo museum by bus under the watch of the Tunisian security forces and hundreds of bystanders. Traffic is flowing again, but bystanders are still here.
One man told me he had come out of sadness and shock. The attack struck at both the political and economic heart of Tunisia. The museum is to many Tunisians what the Louvre is to Paris - a major tourist destination.
This kind of attack on civilians is new here - the last time something similar happened was in 2002. As an act of defiance and national unity, Tunisian politicians are returning to parliament on Wednesday evening. But many Tunisians had hoped that 2015 would bring a more prosperous and stable future.
Anti-terror demonstrations were reported in central Tunis on Wednesday, with crowds waving flags and condemning the attack.
'Tanks rolling in'
A museum employee told Reuters the two attackers "opened fire on the tourists as they were getting off the buses before fleeing into the museum".
Eyewitness Yasmine Ryan told the BBC she saw "helicopters flying overhead" and "tanks rolling in" as the security situation unfolded.
The attack is a huge blow for Tunisia's tourism industry and its government, which only emerged at the end of a long political transition several months ago, the BBC's Arab affairs editor Sebastian Usher says.
Islamist militants have tried to derail the democratic transition, which, although fragile, remains the most positive result of the Arab Spring in the Middle East, our correspondent adds.
The Bardo National Museum
- Tunisia's largest museum, built in a 15th Century palace
- Contains 8,000 works, including one of the world's largest collections of Roman mosaics
- Some items in the collection are more than 40,000 years old
- A new wing was added in 2009, doubling its size
EU foreign policy chief Federica Mogherini offered her condolences (in French) to the victims' families, and said the EU would "fully support Tunisia in the fight against terrorism".
US Secretary of State John Kerry said the US "stands with the Tunisian people at this difficult time" and would continue to support the government's "efforts to advance a secure, prosperous, and democratic Tunisia".
Tourism is a key sector of Tunisia's economy, with large numbers of Europeans visiting the country's resorts.
In 2002, 19 people, including 11 German tourists, were killed in a bomb blast at a synagogue in the resort of Djerba. Al-Qaeda said it had carried out that attack.
Concerns about security in Tunisia have increased in recent months as neighbouring Libya has become increasingly unstable.
A large number of Tunisians have also left to fight in Syria and Iraq, triggering worries that returning militants could carry out attacks at home.