Chilling CCTV footage has emerged showing a suicide bomber calmly patting a child on the head and shoulder moments before he walked into a packed Sri Lankan church, unleashing carnage.

The images capture the final seconds of serenity before the calm was shattered with a series of coordinated attacks that would between them claim more than 350 lives at six sites across Sri Lanka.

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Security camera footage from outside St Sebastian's Church in Negombo shows a bearded man in a light blue shirt, navy trousers and sandals appear on a pathway and cross a courtyard outside the church.

As he approaches his target, he passes a young girl and briefly pats her head and shoulder, with the young girl glancing up at him as he continues on towards the church.

The girl is the granddaughter of Dilip Fernando. The 66-year-old retiree had come to St Sebastian's for Mass, but seeing how crowded it was, decided to try and find a service elsewhere.

That decision probably saved their lives. Some of his family, including his granddaughter, stayed but were outside the church and survived.

Inside, the service was drawing to an end and the hundreds who crammed in to celebrate Easter Mass were preparing to leave.

Those who could not find a seat in the packed church had gathered outside to listen, some holding children in their arms.

Captured from another camera, the suspect can be seen leaning forward while walking, the straps of his backpack fastened across his waist and chest, indicating the weight of the deadly load.

Those gathered outside the church scarcely give him a second glance, while inside, the pews are packed with worshippers, young children fidgeting and ceiling fans whirling against the heat.

The suspect walks past two open doors connecting the church to the covered passageway outside, and enters the church through the third.

He takes a few steps towards the central aisle and the altar.

Then the footage stops.

The bomb he detonated ripped through the church, killing dozens in what is believed to be the deadliest attack in Sunday's carnage.

Fernando's extended family, who were sitting outside the church, said they had seen the bomber.

"At the end of the mass they saw one young man go into the church with a heavy bag," Fernando said.

"He touched my granddaughter's head on the way past. It was the bomber."