Fifty-six people, including two Kenya Police reservists, were shot dead in the coastal town of Mpeketoni by a gang of 18 heavily-armed men dressed in full combat gear and balaclavas.
The Al-Shahbab terror militia has claimed responsibility for the attack, saying it was revenge for Kenya's "brutal oppression of Muslims in Kenya through coercion, intimidation and extrajudicial killings of Muslim scholars”.
It also condemned the “Kenyan military’s continued invasion and occupation of our Muslim lands and the massacre of innocent Muslims in Somalia”.
Witnesses said gun battles lasted for more than eight hours as buildings and cars, both private and government-owned, were set on fire.
The town is on the mainland near Lamu Island, a famous tourist resort and Unesco World Heritage site.
The attack saw President Uhuru Kenyatta convene an urgent meeting with his top security managers and officers. He later dispatched Interior Cabinet Secretary Joseph ole Lenku to the scene - 10 hours after the attack.
Unconfirmed reports said the raid was linked to a land dispute between rival communities in the remote area.
"I heard them shouting in Somali as they fired. I lost two of my brothers, and I escaped,” said John Waweru.
Ole Lenku emerged from a security meeting to describe the attackers as bandits who had crossed a "red line".
KDF spokesman Major Emmanuel Chirchir said the attackers were "likely to be al-Shabab".
Kenya has suffered numerous militia attacks since 2011, when KDF entered Somalia to combat the al-Qaeda-linked al-Shabaab group.
This is the most deadly attack in Kenya since at least 67 people were killed during the siege by an al-Shabaab suicide squad at Nairobi's Westgate Shopping Mall last September.
A Mogadishu-based journalist yesterday tweeted that an al-Shabaab official had confirmed to him the al-Qaeda-affiliated militia group carried out the Mpeketoni attack in revenge for KDF's presence in Somalia.
Tweeting as Live From Mogadishu @AbdulazizHOA, the Mogadishu-based journalist, who specializes in al-Shabaab activity, said that the terror official told him the attack was revenge for Kenya's continued harassment of Muslims on and outside its soil.
"Al-Shabaab official told me: Kenya continued harassing Muslims in and outside its soil. We will keep striking them and more sophisticated. Mpeketoni attack was in revenge to KDF continuation of massacre inside Somalia. They kill innocent with strikes. Our targets were infidels' bases in Mpeketoni and we executed our mission successfully. I say their bodies are everywhere, countless," some of the tweets shared between AbdulazizHOA and al-Shabaab's Twitter account, HSMPRESS, read.
Another tweet claimed that the 18 heavily-armed attackers had also taken hostages.
"The attackers hijacked a van from Witu town, which they used for the attacks. They raided Mpeketoni police station first and opened fire," said a witness.
The raiders arrived at 8pm and started shooting people who were watching a World Cup match.
Those who died included a police reservist attached to Lamu County Deputy Governor Eric Mugo's security detail and many motorists who were caught on the road heading to the Kibaoni area.
Property worth millions of shillings, including an Equity Bank branch, Breeze View Guest House, Winners' Lodge and several other private business premises and over 30 vehicles were also burnt down.
Also destroyed was the Administration Police Divisional Headquarters, which was burnt down, and five vehicles in its yard reduced to ashes.
The gang was armed with machine guns and grenade launchers and unsuccessfully attempted to attack the Mpeketoni police station, which is next to the AP camp.
In one video cafe, nine people were gunned down as they watched a World Cup match.