Thousands of Scotland fans have been enjoying an "amazing atmosphere" in London ahead of the team's game against England later.
The Euro 2020 clash between the teams kicks off at Wembley Stadium at 20:00.
Many fans began travelling down on Thursday, despite warnings they should not come to London unless they had organised somewhere to watch the game.
Tickets to the match are limited because of Covid restrictions and no fan zone has been set up.
Scotland fan Michael McLean, from Inverness, told BBC Scotland he had decided to travel after booking travel and accommodation several months ago.
'Amazing atmosphere'
"It's an amazing atmosphere so far. I think it seems that they've underestimated how many would travel down for the game," he said.
"It's warm here and it feels like a normal football occasion. It doesn't feel like there's a pandemic on which I don't think is good in a way, but it's definitely good to enjoy an occasion after a year-and-a-half."
Mr McLean said the trains had been packed on the way down, with crowds of Scottish fans gathering and chanting outside King's Cross station on Thursday afternoon.
He told
BBC Radio Scotland's Drivetime he had heard London Mayor Sadiq Khan's pleas for fans to stay away, but said much of the travel he had booked was non-refundable.
"£600 in total for all of us would be a lot to lose just to decide not to go down. Fair enough if it was refundable - but I don't want to lose the money."
He added that many fans would be going to Hyde Park as the normal gathering place of Trafalgar Square had been as reserved as a fan zone for key workers.
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image captionTickets to the match itself have been limited because of Covid restrictions
Nick King from Network Rail told
BBC Radio's Good Morning Scotland programme that trains from Glasgow Central were quieter on Friday morning.
"We've had a steady flow of fans trains heading south for the game at Wembley," he said. "The trains to London have been very busy over the last two or three days but we have been a wee bit quieter today.
"We are still telling passengers if you are heading for London please book your ticket in advance, please have a seat reservation because the trains are very busy ahead of the match."
He said that a queueing system was in place for the day for football fans both for London and for tonight's game at Hampden between the Czech Republic and Croatia.
Scotland lost their first game 2-0 against the Czech Republic on Monday.
Wembley and Hampden are among 11 venues across Europe hosting the games in the tournament, which was delayed for a year because of the pandemic.
'Conservative estimate'
Scotland have never made it past the group stage before, but the top two - or possibly three - teams will go through to the knockout phase.
After the match at Wembley, Croatia will visit Hampden on Tuesday for Scotland's second match on home soil.
Emma Best, a member of the London Assembly and the Conservative group's spokeswoman on health, said she believed an estimate that 20,000 Scotland fans would travel might turn out to be a "very conservative" estimate.
"The Tartan Army were everywhere - you'd be forgiven for thinking the game was today," she said.
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image captionLondon Assembly member Emma Best said the atmosphere created by the Tartan Army had been "really good-natured"
Asked about plans for fans to gather unofficially in Hyde Park, Ms Best said: "I think it is concerning and we do need to do what we can to control the Delta variant.
"Like it or not, Scottish fans - and we must say English fans as well from across the country - will be in London in their thousands and so we do need a plan to manage that as carefully as possible."
But Ms Best added that the Tartan Army had been creating a "really good-natured atmosphere" throughout the city.
"The Boogie song is being played everywhere I've been walking. It's a lovely atmosphere and you could be forgiven for thinking we are not still in the midst of a pandemic, and I I think that sums it up.
"What a shame that we can't have this match in normal times and really enjoy this occasion, but we are where we are and I do hope it's as safe as it can be."
Wembley will be open at 25% capacity for the match, with supporters having to show either proof of vaccination or a negative lateral flow test before entry.
Pubs and bars in London are also expected to limit numbers allowed in