A teenager murdered the day after his 16th birthday was killed in an unprovoked attack after he refused to take part in a fight.

A manhunt is under way to track the killer responsible for cutting "gentle giant" Jimmy Mizen's neck with glass after a row as he went to buy his first lottery ticket with his brother.

Jimmy was near his home in the Three Cooks bakery in Lee, south-east London, when a youth challenged him to a fight around 11.50am on Saturday. After Jimmy declined to fight, the attacker smashed down the bakery door and picked up an advertising board to wield inside.

Detective Chief Inspector Cliff Lyons, senior investigating officer, said a fight broke out in which Jimmy got a fatal neck injury.

He died at the scene in front of his brother and became the 13th teenager to be murdered in London so far this year. Mr Lyons said the attack on Jimmy - a 6ft 4in member of Sidcup Rugby Club - was not gang or knife-related.

He said: "Jimmy was a person of immaculate character, as is the Mizen family, and he is a victim of an entirely unprovoked and vicious attack and my heart and condolences go out to the Mizen family."

Police were keeping an open mind about who and where the suspect was, as addresses were searched on Saturday in connection with the inquiry. A police spokesman said a "significant witness" they wanted to speak to in connection with the attack had come forward.

Police had earlier said they were keen to trace a white middle-aged man who witnessed a significant amount of the attack. The man will now be interviewed by officers.

Earlier, around 250 people left standing room only for Mass at Our Lady of Lourdes Catholic Church in Lee, where Jimmy had been an altar boy as a child and had been baptised. Many of the congregation were in tears after a two-minute silence was held at the service, attended by Jimmy's parents, Barry, 56, and Margaret, 55, and other relatives and friends.

One of the altar boys at the Mass was Jimmy's eight-year-old brother George. Jimmy's other brothers included Danny, 30, Billy, 28, Tommy, 26, Bobby, 24, and Harry, 18, and his two sisters were Joanne, 35, and Samantha, 21. Parish priest Father Edward Perera told the congregation: "Yesterday when I spoke to Margaret and Barry, Margaret said something very powerful.

"She said, I will be there in the morning for Mass because I need my faith. The parish is my family and I need the support of the family'."

Afterwards, Father Edward said the service had been deeply overwhelming for him. He said: "I have never seen a parish so numbed and shocked." Jimmy, who lived with his family in Dallinger Road, Lee, was to finish school today to embark on an apprenticeship.

Mr Mizen, vice-chairman of governors at Jimmy's school, St Thomas More Catholic comprehensive in Eltham, described his son as a "gorgeous and loving boy" with a sense of decency who adored his brothers and sisters and "wasn't afraid of hugs".

Mr Mizen, who runs a shoe repair shop in Sidcup, said: "We loved him and he loved us back. He was very proud of his family." Mrs Mizen said: "He was the loveliest, gentlest giant and loved by everyone."

Meanwhile, London Mayor Boris Johnson joined thousands of Christians praying for an end to violence in the wake of the deaths of two teenagers in the capital.

He was speaking at the Global Day of Prayer held at Millwall Football Club yesterday, telling crowds: "In the last few days we have seen the deaths of Lyle Tulloch and Jimmy Mizen and I'm sure their parents will be in our prayers."