Spanish police have confirmed that they are on high alert over an possible attack ahead of Real Madrid's Champions League clash with Manchester City on Tuesday night, after Islamic State issued a chilling threat to all four Champions League quarter-finals.

On Monday ISIS launched a threat against all four stadiums hosting quarter-final matches this week, including Arsenal's Emirates Stadium and Real Madrid's Santiago Bernabeu.

The Al Azaim Foundation, the outlet which is often responsible for spreading messages from ISIS, published a chilling poster with the message 'Kill them all' alongside the names of all four stadiums hosting the matches. That has prompted beefed up security in the Spanish capital, where two of the games are being played.

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Spanish authorities says that “in terms of preventing the terrorist threat, the State Security Forces and Bodies have activated all their early warning and protection systems, as well as all their response mechanisms ready and willing.”

Over 2,000 agents from the National Police and Civil Guard are set to operate a surveillance in Madrid over the next 24 hours, with Atletico Madrid facing Borussia Dortmund there on Wednesday. Arsenal face Bayern Munich at the Emirates Stadium in north London on Tuesday night, with Paris Saint-Germain hosting Borussia Dortmund in the French capital on Wednesday.

The threat comes less than three weeks after Islamic State attacked the Crocus City Concert Hall in Moscow on March 22, when armed men stormed the building and fired guns at the audience. Some 143 people died and around 200 were injured.

A week later the ISIS-linked media outlet Sarh al-Khilafah published an image calling for an attack on Bayern Munich's Allianz Arena during the Bayern vs Borussia Dortmund Bundesliga clash, which thankfully passed off without incident after security around the stadium was increased.

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