* Belaid's killer identified, on the run - PM-designate
* Larayedh says 4 suspects detained, all radical Salafis
* Belaid's killing ignited worst unrest since Ben Ali's fall
* Ennahda says assassins must be severely punished (Adds Ennahda party statement)
By Tarek Amara
TUNIS, Feb 26 (Reuters) - Tunisian police have identified the killer of opposition leader Chokri Belaid as a member of a radical Islamist Salafi group who is on the run, Prime Minister-designate Ali Larayedh said on Tuesday.
Larayedh, who remains Interior Minister until his government is formed, told a news conference police had arrested four accomplices who are also ultra-orthodox Salafis.
The assassination of secular politician Belaid on Feb. 6 provoked the biggest street protests in Tunisia since the overthrow of strongman Zine al-Abidine Ben Ali two years ago.
A security source said on Monday a Salafi had been arrested in connection with the killing, while Tunisia's Express FM radio cited a senior security official as saying police had arrested three Salafis, including a police officer, over the murder.
"Now we have identified the killer of Belaid and he is on the run. The police are looking for him," Larayedh said.
One of the arrested suspects had accompanied the gunman who shot Belaid outside his home before escaping on a motorcycle, he said, adding that the group had mounted surveillance of Belaid's home and a nearby square for several days before the attack.
Hundreds of bystanders watched on Tuesday as two of the detained suspects re-enacted the shooting at the scene amid a heavy security presence, local media said.
The Interior Minister did not confirm the Express FM report that one of those detained was a police officer.
"Identifying the killers of Belaid reinforces confidence in the judiciary and in the neutrality of security (forces)," said Larayedh, who belongs to the moderate Islamist Ennahda party.
After his announcement, Belaid's widow Basma said it was still not clear who had orchestrated her husband's assassination, which was the first in Tunisia for decades.
"It's good to know who killed Chokri, but it is very important to know who gave the order because it was a very organised crime," she told France's Europe 1 radio in Paris.
No one has claimed responsibility for the killing. Ennahda has denied accusations by some, including Belaid's brother, that it was involved in the assassination, which it has condemned.
In a statement, Ennahda leader Rached Ghannouchi denounced such accusations against his party and called for Belaid's assassins to be severely punished for a "heinous crime which ... endangered civil peace and co-existence among Tunisians".
Ennahda "calls on all political parties, civil society associations and all Tunisians committed to the revolution to work in solidarity and cooperation so as not to give any chance to those who wish to drag the country towards violence and in-fighting," the statement said.
In a dig at Salafis, it also urged young Tunisians to "promote moderate thought and balance which eschews takfir (accusing another Muslim of apostasy) and violence".
Salafis prevented several concerts and plays from taking place in Tunisian cities last year, saying they violated Islamic principles. Salafis also ransacked the U.S. Embassy in September during worldwide Muslim protests over an Internet video.
A hardline Tunisian Salafi group, Ansar al-Sharia, which is said to have links to al Qaeda, is led by Saifallah ben Hussein, who is better known as Abu Iyadh. He is wanted by police on incitement charges in connection with the U.S. embassy attack.
Many Ansar al-Sharia members, including Abu Iyadh, were jailed in Tunisia during the Ben Ali era.
Secular groups have accused the Ennahda-led government of a lax response to Salafi attacks on cultural venues and individuals in recent months.
After Belaid's death, Prime Minister Hamadi Jebali tried to restore calm by proposing an apolitical cabinet of technocrats to organise a parliamentary election, but resigned after opposition from within his own Ennahda party scuppered the plan.
On Friday, President Moncef Marzouki asked Ennahda's nominee Larayedh to form a new government within 15 days.
The so-called Jasmine Revolution that toppled Ben Ali in January 2011 was the first of several Arab uprisings.
Tunisia's political transition has been more peaceful than those in neighbouring Egypt and Libya, but tensions are running high between Islamists elected to power and liberals who fear the loss of hard-won liberties.
While Islamists did not play a major role in the Tunisian revolt, the struggle over Islam's role in government and society has emerged as one of the most divisive political issues.
Salafis, not all of whom espouse violence, want a broader role for religion in Tunisia, alarming secular elites who fear they will seek to impose their strict views at the expense of individual freedoms, women's rights and democracy. (Reporting by Tarek Amara; Editing by Alistair Lyon)