The UK and Europe are on alert after Islamic State (Isis) threatened to attack this week's Champions League matches.
Security has been ramped up across the continent ahead of four games taking place on Tuesday and Wednesday.
Real Madrid host Manchester City in the first leg of their quarter-final this evening before Atletico Madrid take on Borussia Dortmund on Wednesday.
Arsenal also play Bayern Munich in their first leg in north London tonight, with PSG hosting Barcelona at the Parc des Princes 24 hours later.
Champions League Isis threat: The Spanish government are on high alert
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A graphic poster was released on Monday by the Al Azaim Foundation - the media outlet Isis uses to spread its messages.
The poster displayed the names of all four stadiums that will be hosting the first-leg ties.
There is also a picture of a person holding a rifle, with the message underneath reading: 'Kill them all'.
It has raised concerns over the safety of spectators attending this week's huge matches.
Arsenal will have over 60,000 of their own fans at the Emirates Stadium, with no Bayern supporters present due to a Uefa ban.
The Bernabeu has a capacity of over 80,000 for the visit of City, while there will be over 70,000 at the Metropolitano Stadium for Atletico vs Dortmund.
There will be just under 50,000 in attendance for PSG's game against Spanish giants Barca.
Uefa have confirmed that the games are currently set to go ahead as normal. A statement reads: "Uefa is aware of alleged terrorist threats made towards this weekâs UEFA Champions League matches and is closely liaising with the authorities at the respective venues.
"All matches are planned to go ahead as scheduled with appropriate security arrangements in place."
The Metropolitan Police confirm a 'robust policing plan is in place' for Tuesday's game in London.
Deputy Assistant Commissioner Ade Adelekan said: "The UK terrorism threat level remains at âsubstantialâ meaning an attack is likely, and we work closely with colleagues from across Counter Terrorism Policing in planning for events here in London, to take into account any relevant information that could help us to keep those attending safe.
"We're aware of online and media reports in relation to calls to target matches across Europe and here in London.
"However, I want to reassure the public that we have a robust policing plan in place for tonight's match and we continue to work closely alongside the clubâs security team to ensure that the match passes peacefully.
"As ever, we ask the public to remain vigilant, and if they see any that doesnât look or feel right, then report it to police or security staff."
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Champions League Isis threat: A poster was released on Monday on social media
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Key government figures in Spain have taken action in an attempt to thwart any attack in Madrid.
La Vanguardia claims that Spain's Ministry of Interior has activated all their 'alert' and 'response systems'.
A 'high alert level' is in place and an 'extensive security operation' will operate for supporters attending.
Meanwhile, AS reports that the games involving the two Madrid teams have been declared 'high risk' by the Anti-Violence Commission.
It is said that there will be more than 3,000 members of security between the two matches.
Champions League Isis threat: The Metropolitano will host Atletico Madrid vs Borussia Dortmund
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There will be over 8,000 Man City and Dortmund fans in Madrid over the next two evenings.
The Islamic State's last attack took place on March 22 at the Crocus City Hall in Moscow, where 143 people were killed and a further 200 injured.
Several armed men in camouflage stormed the building and fired automatic weapons at people in the audience.
The latest threat on football fans comes after Isis-affiliated media outlet Sarh al-Khilafah announced a plan to attack the Bundesliga match between Bayern and Dortmund on March 30.