The families of two British hostages who were beheaded by Islamic militants have come together in a show of unity at a special service to celebrate the life of one of the murdered men.

Michael Haines called on people to join together and "find a single act of unity" in the wake of the barbaric killing of his brother, David Haines.

Father-of-two Mr Haines was taken hostage in Syria while working for international relief agency Acted in March last year.

He had been been helping refugees in a camp near the Turkish border when he was snatched by Islamic State (IS) militants.

Tributes were paid to him a memorial service in Perth, which was also attended by the family of Alan Henning, who was also killed by IS.

His widow, Barbara, and two children travelled north to attend the service at Perth Congregational Church, which was described as a "service of thanksgiving and celebration for the life of David Haines".

Mrs Henning and Michael Haines stood together and held hands outside the church before going in for the service, which was attended by more than 600 people.

In a joint letter published in the Guardian newspaper they called on people to come together in a bid to defeat the terrorists.

''Together we have the power to defeat the most hateful acts," they said

''We condemn those who seek to drive us apart and spread hatred by attempting to place blame on Muslims and the Islamic faith for the actions of these terrorists,'' the letter stated.

The pair said they have been overwhelmed by messages of support from around the world.

Born in Yorkshire, David Haines was brought up in the Perth area and attended the local academy before serving 12 years as an aircraft engineer with the RAF.

He later took up humanitarian work and assisted aid agencies in some of the world's worst troublespots.

He was in Libya during its civil war in 2011, working as head of mission for Handicap International, which helps disabled people in poverty and conflict zones around the world.

Mr Haines has a teenage daughter, Bethany, from a previous marriage and a four-year-old daughter in Croatia with his widow, Dragana Prodanovic Haines.

Michael Haines said his brother's death had broken the family's hearts but they had been "overwhelmed" by messages of support from the British public and people all over the world.

As the service was a celebration of Mr Haines' life, his family had encouraged people to come dressed in bright colours. One man in the congregation wore a colourful yellow Hawaiian-style shirt and red tartan trousers, while Michael Haines wore a green and purple kilt teamed with a purple T-shirt and black leather waistcoat.

Bethany Haines arrived in a floral dress, clutching a single white rose.

Both she and Michael Haines spoke during the service, sharing some of their memories of the murdered aid worker.

Michael Haines' son, Euan, read a poem, Not How Did He Die, But How Did He Live, in memory of his uncle.

The service also featured several hymns, including Amazing Grace and I Vow To Thee My Country, as well as the song Going Home, by the Scottish band Runrig.

Scottish Finance Secretary John Swinney was among those who attended, going into the church alongside fellow local SNP politician Pete Wishart MP.

Shortly before proceedings got under way, a group of about 20 bikers drove past, revving their engines loudly, before making their way into the church, as part of a tribute said to have been organised by a family friend.

Reverend Gordon Campbell said afterwards that tears had been shed during the service, but there had been laughter too.

He said he had known Mr Haines, who was a former Scout leader, when he was the organisation's commissioner for Perth and Kinross, adding that Mr Haines had also been part of the committee who interviewed him for the position at Perth Congregational Church in 2006.

"It's partly because of David I'm here in the first place," he said.

"He was an adventurous guy, with a sense of fun, but I think he will be remembered above all for his caring disposition and his concern for other people, and that was something we were reflecting on in the service."

He added: "Tears and laughter are sometimes not as far apart as we imagine, and they were both in church this afternoon.

"But right at the beginning we said 'Despite what some of you might think, you are allowed to smile and you are allowed to laugh in church' and, yes, there was laughter."

He went on: "It was lovely to have so many people there supporting the family, people from different aspects of David's life.

"The service was billed as one of celebration and thanksgiving and we tried very much to be uplifted by David's achievements, David's life, David's legacy, but we were also trying to think how we let that legacy live on in our lives."

The service had a "message of peace and reconciliation", he said, adding that the Muslim Council for Scotland had been represented in the congregation.

Prime Minister David Cameron described the beheading of Mr Haines as ''an act of pure evil'' after Islamic State released a video showing his murder last month.

In a message recorded on the eve of the memorial service, Michael Haines said: "David would want us all to remember that extremism, terrorism and the mistreatment of people is not something that just happens to those thousands of miles away.

"It's something that affects us all in the UK too. My brother's killers want to hurt all of us and stop us believing in the very things which took David into conflict zones - charity and human kindness.

"I am calling on all of us - every community, every faith - in the coming weeks and months to find a single act of unity. One simple gesture, one act, one moment that draws people together."

He added: "My brother didn't see other nationalities or religions, he just saw other human beings in need of a real help to get by, or sometimes a lot of help to live to see another day.

"This is how my family will remember him, and we hope you all will too - a man full of kindness, open and caring, willing to cross the road to help others."

He stated: "I am recording this message in the hope that people across Britain will listen to my family's story and understand that the fight against terrorism and extremism isn't something that happens to other people - it affects us all.

"How we react to this threat is also about all of us. Together we have the power to defeat the most hateful acts.

"Acts of unity from us all will in turn make us stronger, and those who wish to divide us weaker."