Soldiers with assault rifles in combat dress are helping to guard the entrance to Horse Guards Parade in central London's Whitehall.
The presence of the troops comes as the US announced increased security at federal government buildings in Washington and other cities.
The move follows the shooting of a soldier in Canada last week.
Nathan Cirillo, 24, who was shot by Michael Zehaf-Bibeau while standing guard at Canada's war memorial.
Ceremonial soldiers of the Household Cavalry Regiment usually assume sentry duties at Horse Guards Parade but they have now been joined by soldiers in camouflage carrying guns.
The entrance of the popular tourist attraction is where the daily Changing of the Guard ceremony takes place.
A spokesman for the Ministry of Defence said armed troops are patrolling the area to protect the military barracks on Horse Guards Parade, rather than tourists or the prime minister.
It is understood a decision to station armed soldiers at the site would have been taken by a local commander.
Troops with guns will be on patrol "periodically" not permanently, but protection of tourists remains the Met's responsibility, added the spokesman.
Following the incident in Ottawa, there have been calls for security in Westminster to be stepped up.
House of Commons officials said they "remained vigilant" but former Cabinet minister Peter Hain called for a review of security in Westminster.
Mr Hain, who introduced tougher security measures as Commons leader in 2004 following a series of breaches in the chamber, said it would be "prudent to review everything" following the Canadian attacks.
"I'm sure the head of security will now be urgently reviewing our procedures," the former Northern Ireland secretary said.
"I believe our procedures are much more rigorous than in 2004 when security was a joke."
A review of David Cameron's security is being carried out after a scare earlier this week when a member of the public ran into the prime minister and his protection team in Leeds.
The prime minister has insisted he has confidence in his police bodyguards despite the apparent security breach, but the Metropolitan Police (Met) said it would carry out a review.
The Met specialist protection command, which is responsible for the personal protection of the prime minister, was examining the incident.