Croatia had been desperately poor, a shadow of the team who carved Argentina apart in the group stage, but started to get a foothold in the game as the match rolled past the hour mark and Luka Modric's influence grew.
Asked if Croatia could beat France in the final on Sunday, his girlfriend Marina said: "Of course". Mandzukic scored in extra time to make it Croatia 2-1 England.
Southgate said the players were deeply disappointed, but that his young team could achieve great things in the years to come, with the European Championship just two years away.
England took the lead early on with Kieran Trippier scoring the goal in the first half.
It was heartbreak for England, but joy for those who backed Croatia at a high of [40.0] to win in extra-time in the Method of Victory market.
The players in question, midfielders Dele Alli and Eric Dier, and forward Raheem Sterling, were seen wearing branded ankle support socks over their official Nike-branded socks during England's 2-0 quarter-final win over Sweden.
It would certainly be a massive disappointment for Croatia to be infront of an empty stadium - especially after their World Cup heroics.More news: LONDON FALLING: US Embassy Warns Americans of 'VIOLENT' PROTESTS During Trump Visit
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The party spilled out into the street after the game wrapped up, where some fans lit off red, white and blue smoke flares as they sang popular chants.
"The players and the squad, everybody involved in the World Cup, have a reason to come back home with pride after giving everything and making the country proud".
Croatia striker Mario Mandzukic scored the victor in the 109th minute, sweeping home a left-foot shot after England had allowed him too much space in the penalty box.
"We're gutted", said Harry Kane, still on course to finish as the tournament's top goalscorer with six strikes in Russian Federation. The ball bounced twice, Mandzukic ran onto it and one-timed a low, left-footed shot to Pickford's left.
"There was a spell in the second half when they got the goal and their tails were really up and we had to weather the storm and lost our way a little bit".
That was enough to silence the entire city as it became apparent that England's World Cup campaign would end at the semi-final stage, yet again. This is an awesome achievement for us - it's a dream come true after such a long time.
"But the team will be stronger for that and as we reflect over the next few days, there will be a lot more positives".