Ninety-six Liverpool supporters died in the crush at Sheffield Wednesday's Hillsborough Stadium on April 15, 1989.
The Kop and the Centenary stands at Liverpool's Anfield Stadium were opened early for the official memorial service, which is due to begin at 2.45pm.
As numbers grew, part of the main stand was also opened to the public.
At 3.06pm, the exact time the referee blew the whistle and abandoned the FA Cup semi-final between Liverpool and Nottingham Forest, a two-minute silence will be held throughout Merseyside and in Nottingham's Old Market Square.
After consultation with the victims' families it was decided there would be no formal ceremony in Sheffield.
The Leppings Lane end of the Hillsborough ground, where the crush took place, was opened up earlier today for people to visit and pay their respects.
Hundreds of floral tributes, scarves and football shirts of all colours were laid outside the Hillsborough memorial and tied to the Shankly Gates outside the Kop on Anfield Road.
Groups of people stood, hugged and some wept as they looked at the names of those who died in Britain's deadliest sporting disaster.
At the centre of the memorial burns an eternal flame, signifying they will never be forgotten.
Sue Joyce, 43, from West Derby, Liverpool, said: "We've come here today to show the victims and the families of those who died that we have not forgotten what they have suffered.
"It may be 20 years since the disaster took place but those that were there will always be in the thoughts of every Liverpool fan around the world."
As the families of the victims took their places on the Kop, the crowd of up to 25,000 people gave them a huge round of applause.
There were also loud cheers and clapping for a group of Celtic fans who laid two banners on the Anfield turf emblazoned with "Justice for the 96" and "You'll never walk alone".
Club officials then took their seats, followed by members of Liverpool's Academy.
As Pepe Reina led the first team out, there were huge cheers and applause.
Rafa Benitez followed with his wife, Montse, coach Sammy Lee, Jamie Carragher and Steven Gerrard.
There were also cheers and applause for Everton's manager, David Moyes, and Kenny Dalglish.
Opening the service the Bishop of Liverpool, the Rt Rev James Jones, told the crowd the Queen had sent a message that her thoughts and prayers were with them.
The Bishop said the tragedy "broke the heart but not the spirit" of the community.
He said: "On this the 20th anniversary of the tragedy at Hillsborough, which broke the heart but not the spirit of our community, Her Majesty the Queen has asked me to say that her thoughts and prayers are with us and all those affected by the tragedy.
"For many here today it seems still like yesterday. Those we lost always in our minds.
"Never a day passes without a thought of what their tomorrow might have been, without that longing for justice for their sake as well as for ours."
As the service continued, a candle was lit in memory of each victim and their names were read out.
People continued to pour into the stadium and stewards opened the Anfield Road stand.
The crowd, now estimated at up to 30,000, stood solemnly as the two-minute silence was held.
At the end of the silence, church bells from around Liverpool could be heard ringing out 96 times.
Dalglish, who was Liverpool's manager when the disaster took place, read from the Bible, Lamentations of Jeremiah.
Margaret Aspinall, vice-chairwoman of the Hillsborough Families Support Group, gave the second reading from the Letter of St Paul to the Romans.
In Sheffield, around 300 people attended a brief memorial service at Hillsborough stadium.
Many of those at the ground broke down in tears as they observed a two-minute silence at 3.06pm.
The short service was held by a vicar in front of the memorial at the main entrance to the stadium's South Stand.
The memorial was draped with Liverpool flags and red and white flowers, while a carpet of flowers, flags and other tributes had been laid at its base.
Many people spent time looking at the tributes before visiting the Leppings Lane end of the stadium, which was opened to members of the public to pay their respects.
Some 2,000 Liverpool and Forest fans packed into Nottingham's Old Market Square to remember those who died at Hillsborough.
Many held up Liverpool and Forest scarves during a two-minute silence, which was followed by the playing of You'll Never Walk Alone.
The crowd then broke out into spontaneous applause.
During the two-minute silence, a church bell tolled 96 times in memory of the fans who died.
Jeremy Nicholas was commenting for BBC Radio Nottingham when the disaster unfolded.
Nicholas, 46, said today: "It was very emotional today.
"We were reporting on the game when we realised pretty early on there was a major problem.
"We could see there were too many people and we could see that something terrible was going on.
"Something that really stuck in my mind was this policeman who ran on to the pitch and told the referee to stop the game.
"We were there to report on a football match, but suddenly there was a tragedy unfolding in front of us.
"I remember people ripping down some advertising boards and seeing people being carried out.
"We didn't know whether they were dead or alive and I kept saying: 'It's not the Forest fans.' I felt really bad for saying it."
Sheila Cartman-Miles, 61, had two sons at the game supporting Forest.
She said: "I was proud to be part of the service today.
"I felt for those parents who had sons and daughters who never came back.
"Every time I hear You'll Never Walk Alone, it comes back to me, it's in my heart."
Fred Fisher, 70, was at the game with his son, Martyn.
He said: "I can always remember this Liverpool supporter who was about 13 and he came across the pitch and he shouted: 'What are you cheering for, there are people dying.'
"We didn't know what was happening. People were breaking down the barriers and carrying out people."
He added: "Today was very emotional for us. You feel it's part of you and I wanted to let the people of Liverpool know how sorry we are for them."
Paul Denovellis, 46, from Nottingham, said: "I came here today because I wanted to pay my respects to the fans who died and their families and all the fans of Liverpool as well."
Thousands stopped in Liverpool city centre at 3.06pm to pay their respects by observing two minutes silence.
Workers had poured out of offices to join shoppers standing still, heads bowed in the mid-afternoon sunshine.
All across the city bells could be heard ringing for the 96 victims.
In the city's main streets and shopping thoroughfares public transport stopped and motorists pulled over to take part in the impeccably-kept silence.
In Exchange Flags - a public square behind the town hall in the heart of the city's business district - hundreds of men, women and children formed an impromptu circle of solidarity.
For some the emotion was too much.
They wiped tears from their eyes as thoughts were concentrated on the scores of families torn apart by Britain's worst sporting disaster.
When the silence finally ended spontaneous applause rang out.
People patted each other's backs, shared a word and returned to their day.
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