British Muslims have expressed fury and anguish in the wake of the brutal killing of Alan Henning by Islamic State (Isis) militants, as the family of the Salford taxi driver said they were "numb with grief" at news of his murder.

Many in the UK Muslim community had been hoping the aid convoy volunteer might be freed on the eve of the Islamic festival of Eid al-Adha. Vigils had been held in his home town and more than 100 high-profile Islamic leaders had appealed for him to be released. But the posting of a gruesome video on Friday night, appearing to show his beheading, ended hopes and unleashed a torrent of condemnation.

Harun Khan, deputy secretary general of the Muslim Council of Britain – the largest Islamic organisation in the UK, representing more than 500 organisations – said: "Yesterday was a huge day of significance because it was the day when people will seek forgiveness and salvation. It's a time of peace, this was really shocking to a lot of people. If Isis really wanted to win the propaganda war, they would have released Alan. They are not really Islamic: nobody recognises them, and they are hijacking the religion."

Kasim Jameel, from Bolton, who was with Henning on the convoys and first interested him in helping the people of Syria, said: "I'm totally heartbroken. What can you say? When you lose someone so important to you, you can't put it into words. Everyone who knew him from the convoys just can't stop crying – grown men with beards. We keep expecting him to come round the corner, and say, 'I was only joking'.

"He is the best of the best, a pure soul, he is in the best paradise. It's time for him to rest, he's done a lot of work, God's called an angel back."

Muslims pray for murdered aid worker Alan Henning in Manchester central mosque. Photograph: Oli Scarff/AFP/Getty Images

Henning's widow Barbara said she and their two children were numb with grief and that his murder had been the "news we hoped we would never hear". In her statement she thanked everyone who had supported the family: "During this ordeal we have relied heavily on the support of many people. Support from the government, Foreign Office and Greater Manchester police has been there from the start and has meant we were able to get through the most awful times. We always knew Alan was in the most dangerous of situations but hoped he would return home to us. That is not to be.

"I want to thank everyone who campaigned for Alan's release, who held vigils to pray for his safe return and condemned those who took him. Your efforts were a great support to us, and we take comfort in knowing how many people stood beside us in hoping for the best. We as a family are extremely proud of him and what he achieved and the people he helped."

The decision by the jihadis to release the video shortly after sunset in the UK, the moment that marked the start of Eid al-Adha, has been treated by many Muslims as a deliberate provocation. Julie Siddiqui, former head of the Islamic Society of Britain, said her mosque in Slough, Berkshire, was packed on Saturday morning with 900 people "appalled" at the murder.

"Everybody is sickened by this," she said. "Why did they decided to announce it when they did? Some of us, naively, thought Alan might even have been released on Friday. They chose a day of reflection, when many were thinking of peace forgiveness, thinking about others. There is a genuine feeling that they did it deliberately, it seems too much of a coincidence."

David Cameron pledged that the UK will use "all the assets we have" to eradicate the fighters responsible for the "senseless" murder of Henning. But there were further ominous developments in Syria, where Isis appeared to be on the verge of taking the Kurdish town of Kobani. Terrified citizens who had not already fledtowards the Turkish border were reportedly trying to arm themselves or simply hiding in their homes.

Speaking after a hastily convened meeting with senior defence, Foreign Office and intelligence chiefs, including the head of MI5, at his official country residence Chequers, Cameron described Henning as a man of "great peace, kindness and gentleness".

But Henning's brother-in-law, Colin Livesey, said the British government could have done more "when they knew about [his captivity], months and months ago". Henning was kidnapped on Boxing Day last year, just half an hour after entering Syria, driving a vehicle full of clothing and food aid for Muslim refugees.

A Muslim man prays during a prayer session for murdered aid worker Alan Henning in Manchester central mosque. Photograph: Oli Scarff/AFP/Getty Images

Film-maker Bilal Abdul Kareem, who helped in the negotiations when he was first captured, also accused Cameron of not doing enough to help. He added that Isis knew the strength of opposition to the murder, but chose to "spit in the Muslims' eye to show them who is boss".

He revealed that a representative of al-Qaida had appealed to the fighters holding Henning to let him go just four days after they picked him up. "Nobody outside of Isis thought this was a good idea. Nobody thought that it was OK to do this," he said. "The al-Qaida representative went to go down and try to talk to them, and [when] he returned his face was different. He said something to the effect that these guys are really being difficult, really being tough, but they did say they were going to release him. Everybody was anticipating that but that never happened."

Suleman Nagdi, of the Federation of Muslim Organisations, heard the news after returning from a vigil for Henning in Leicester. "We have to disassociate from the two words Islamic and State," he said. "There is nothing Islamic about these individuals, nor is it a state. My question to these young people [who might be sympathetic to Isis] is simple: who is living closer to the message of the Qur'an? Is it Isis, or is it somebody like Alan Henning?"