Kenya on Sunday began three days of national mourning for the 148 people, mostly students, massacred by Somalia's Shebab militants at a university in Garissa.

Easter church services throughout the country included prayers for the victims of Thursday's attack, with flags also at half mast.

Although President Uhuru Kenyatta has vowed to retaliate "in the severest way possible", there have also been calls for national unity.

Kenyatta said people's "justified anger" should not lead to "the victimization of anyone" -- a clear reference to Kenya's large Muslim and ethnic Somali minorities.

Authorities meanwhile announced that they had identified one of the four dead Shebab gunmen as an ethnic-Somali and Kenyan national who was a A-grade pupil and law graduate -- highlighting the Al-Qaeda-linked Shebab's ability to recruit within Kenya.

Interior ministry spokesman Mwenda Njoka said it was "critical that parents whose children go missing or show tendencies of having been exposed to violent extremism report to authorities".

The militants attacked the university at dawn, and lined up non-Muslim students for execution in what Kenyatta described as a "barbaric medieval slaughter".

The massacre, Kenya's deadliest attack since the 1998 bombing of the US embassy in Nairobi, claimed the lives of 142 students, three police officers and three soldiers.

"The terrible events in Garissa are still fresh in our minds and heart, but today is a day for new hope," Kenyan Anglican Archbishop Eliud Wabukala told a packed cathedral in Nairobi, as armed soldiers patrolled outside.

"These terrorists want to cause scare and divisions in our society, but we shall tell them, you will never prevail," he said.

Top Muslim leader Hassan Ole Naado also offered his condolences.

"Kenya is at war, and we must all stand together," Naado said, deputy head of the Supreme Council of Kenya Muslims, saying the organization was helping to raise money for the funerals of those killed and medical costs of the scores wounded.

"We deeply feel the pain of the loss of young lives," he added in a statement, warning that the Shebab was aiming to "create religious conflict".

Political and religious leaders have condemned the attacks.

Pope Francis called the killings "senseless brutality", while the Cairo-based top Sunni Muslim body Al-Azhar has condemned the "terrorist act committed by Somalia's Shebab".

On Saturday, the Shebab warned of a "long, gruesome war" unless Kenya withdrew its troops from Somalia, and threatened "another bloodbath".

The Shebab also carried out the Westgate shopping mall attack in Nairobi in September 2013, a four-day siege which left at least 67 dead.

Hours after the Shebab's warning, police in Garissa paraded four corpses of the gunmen piled on top of each other face down in the back of a pick-up truck followed by a huge crowd.

Five men have also been arrested in connection with the attack, including three "coordinators" captured as they fled towards Somalia, and two others in the university.

The two arrested on campus included a security guard and a Tanzanian found "hiding in the ceiling" and holding grenades, the interior ministry said.

A $215,000 (200,000 euro) bounty has also been offered for alleged Shebab commander Mohamed Mohamud, a former Kenyan teacher said to be the mastermind behind the Garissa attack.

Forensic investigators aided by foreign experts continued to scour the site where one student shocked security forces on Saturday by emerging unharmed from a wardrobe where she had hidden for over two days.

The remaining 600 traumatized student survivors from the now-closed college have since left Garissa for good, boarding buses for the home towns around the country.

Over 200 family members of those killed continue their agonizing wait for the remains of their loved ones at the main mortuary in Nairobi.

There has been growing criticism in the media that critical intelligence warnings were missed, and that special forces units took seven hours to reach the university, some 365 kilometers (225 miles) from the capital.

Foreign Minister Amina Mohamed defended the response, telling AFP that "fighting terrorism... is like being a goalkeeper. You have 100 saves, and nobody remembers them. They remember that one that went past you."