Thousands of security officers will be deployed in the two major cities for the New Year celebrations in order to prevent terrorist attacks.
London plans to beef up its police presence and closed down some roads for New Year's Eve after a year marked by repeated attacks.
The Metropolitan Police said on Thursday there's no specific threat to the city's massive celebration, which is focused on a fireworks display over the River Thames, but says the public should be vigilant.
Superintendent Nick Aldworth said revelers should expect to see armed police and vehicle barriers.
Police are asking the public to be patient on New Year's Eve because there will be many checkpoints in place, he said.
Police say tickets for London's fireworks display have all been sold out and that those without a ticket should consider watching the event on television.
Istanbul gears up for celebration
Istanbul will deploy over 40,000 security officers for the night of New Year's Eve, one year after a deadly attack on a nightclub claimed 39 lives, its governor said on Thursday.
Following the attack on the Reina nightclub by Daesh, celebrations in the city are expected to be low-key with several popular areas in the city forbidding public events.
Turkish police have detained dozens of suspects nationwide in last few days, although there has been no indication of any concrete link to the New Year.
Istanbul governor Vasip Shain said that 37,000 policemen and 4,000 members of the gendarmerie and coastguard would be deployed on the night of December 31-January 1 to ensure security.
Entertainment venues had been told to have their own private security officials on hand and if the precautions were inadequate then police would be deployed, he said.