There will be an annual Stephen Lawrence Day to be held on the anniversary of the teenager's murder, 22 April, the prime minister has announced.

Theresa May made the announcement at a memorial service to mark the 25th anniversary of the racist murder that shocked Britain and led to the Metropolitan police being described as institutionally racist, the Guardian reported.

Stephen's mother Doreen welcomed the announcement. 'I feel honoured [May] has recognised the changes that have been made in Stephen's name and the changes that are still needed,' she said.

She added that Stephen Lawrence Day would be 'an opportunity for young people to use their voices and should be embedded in our education and wider system regardless of the government of the day'. The first Stephen Lawrence Day will be held next year.

Prince Harry and his fiancee Meghan Markle were among around 800 people who attended the service yesterday afternoon at the St Martin-in-the-Fields church in Trafalgar Square, central London.

It was also attended by the Labour leader Jeremy Corbyn, the London mayor Sadiq Khan, the actor Sir Lenny Henry, the singer Beverley Knight, and the Metropolitan police commissioner Cressida Dick.

Doreen Lawrence and her son Stuart stood at the church's entrance before the service, greeting guests, many of whom were wearing the orange ribbon lapel badges of the Stephen Lawrence Charitable Trust.

Prince Harry and Markle were the last VIP guests to arrive, stopping to talk briefly to the Lawrences, according to the Guardian. Crowds cheered at the arrival of the couple, who are to marry in less than a month.

During the service, Harry read out a letter of support on behalf of his father, the Prince of Wales. It said: 'I remember vividly the profound shock that I felt at [Stephen's] senseless murder, a feeling shared by so many people across this country and beyond.

'I remember, too, just how deeply moved I was by the determination of his family to build something positive from the tragedy they endured and to ensure that Stephen's story did not end with despair, but continued with hope.'

Tributes to Stephen were also sent by May, Corbyn, the home secretary Amber Rudd, and the former home secretary Jack Straw.

Corbyn read from Nelson Mandela's autobiography, Long Walk to Freedom. The late former president of South Africa and anti-apartheid hero and met the Lawrence family at his request while on a visit to London two weeks after the teenager's murder. The meeting helped focus national and police attention on the crime, with initial arrests being made the following day.

The service opened and closed with music from a gospel choir, and along with hymns, prayers and reflections, Knight sang two songs, 'A Change is Gonna Come' and 'Fallen Soldier'.

Henry interviewed three beneficiaries of the Stephen Lawrence Charitable Trust, which supported more than 2,000 young adults last year with training, mentoring and bursaries.

Doreen Lawrence wrote in an introduction to the order of service: 'I wish for Stephen's name not to be identified by his murder, but by the mark he has left on this country and the wider world and for the role model he was and continues to be for many young people today, even though the majority of them would not have been born at the time of his death.

'It has been a long journey that is not over yet, for there are many injustices that are still taking place today.'

Stephen Lawrence was murdered by a gang of five white youths in Eltham, south-east London, on 22 April 1993. Only two of those, David Norris and Gary Dobson, were convicted of murder almost two decades later. Last week, Scotland Yard said it had no new lines of inquiry to pursue.

The shocking police failures led to a watershed moment in race relations in the UK, and the subsequent Macpherson inquiry concluded that the police were guilty of institutional racism.

Speaking after the service, Corbyn said he had felt 'very honoured' to read from Mandela's autobiography. He added that although the country had come far on racism since Stephen's death, 'we still have a long way to go'. The opposition leader said that there were too many young black men in the criminal justice system, too much Islamophobia and too much antisemitism, adding that racism was 'a poison'. 

The government, he added, needed to resolve the 'appalling' Windrush scandal and the home secretary 'has got to explain how to get out of the problem or get out of the way and let someone else do it'.

Henry told the Guardian the while the service had been a celebration, it was also a 'moment of incredible sadness'. He said: 'There is no finish line to racism. It is an ongoing problem we have to solve, something we have to keep on fighting.'

The comedian said of the Windrush scandal: 'My heart goes out to anyone affected by the scandal. When I was speaking in the service, I told them – Theresa May, Jeremy Corbyn, Sadiq Khan, sitting in front of me – "You guys have got to sort this out", and they all nodded.'

Doreen Lawrence said that the service had been 'amazing' and she was grateful to those who had worked on it, adding: 'I was holding myself together all the way through. It's been a really emotional day.'