The Met Police has issued a warning over the current terror threat level in the capital ahead of the European Championship.
The force has also urged fans to respect coronavirus regulations while watching games.
The tournament, postponed last summer due to the pandemic, is set to kick-off on Friday; England play their first game against Croatia on Sunday.
The Metropolitan Police is calling for fans to remain aware of any potential attacks in the coming weeks.
“The terror threat remains at 'substantial',” said Deputy Assistant Commissioner Laurence Taylor.
“This means that an attack is likely and I am asking the public to play their part toward security by remaining vigilant and reporting any suspicious activity or behaviour to stewards, security staff or police officers.”
The force is also calling for fans to respect coronavirus regulations.
“London remains in a public health crisis and we must all still take action to stick to government guidelines on social distancing and gatherings,” said Mr Taylor.
“Please, only come to London if you have a ticket for a match, fanzone or somewhere safe to watch the match. There are no alternative sites for fans to gather in large numbers and there are limited spaces in pubs and bars.”
In preparation, Met licensing officers have been visiting pubs, bars and other venues showing the matches to talk to staff and train them in how they can help keep their customers safe and deal with any arising disorder and anti-social behaviour, particularly as a result of excessive drinking.
Officers have also advised venues how to prevent and reduce sexual violence and how staff can create safe environments for female customers.
There will be full range of officers from Public Order, the Central Football Unit, Territorial Support Group and Project Servator, Mounted Branch, Dog Unit, Firearms Unit, Air Support, and the Marine Unit to respond to any crime or critical incidents which may occur.
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