A video shared on social media shows several police officers pinning a man to the ground.
The short clip shows the man, wearing blue jeans and a jacket, being restrained on the floor by police officers.
A second video, taken from inside an adjacent shop, also shows police restraining the man.
Other photos show a second man being handcuffed and led away by officers.
This man is dressed in brown and green – possibly khaki. Police appear to be hand-cuffing him before he is led away.
A manhunt is under way for a different man, who is believed to have fled towards an underground station.
Stockholm police have made detained two people following today's attack
Despite the photos, Swedish police have said in a statement no arrests have been made.
It has not been confirmed whether the two men have been seized in relation to today's Stockholm terror attack.
At least four people have died following the attack in the city centre this afternoon.
A truck careered down a pedestrian street before crashing into a department store.
The Stockholm Terror attack – in pictures Fri, April 7, 2017
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The vehicle drove into crowds on Drottninggatan (Queen Street) – one of the city's major pedestrian streets, just before 3pm local time, before crashing into Åhlens Mall
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Truck rams into crowds in central Stockholm
Three deaths have been confirmed by officials – although some media in Sweden are reporting higher death tolls.
Several others are injured.
Stockholm police seized the man following the truck attack
The vehicle drove into crowds on Drottninggatan, one of the city's major pedestrian streets, just before 3pm local time.
It then crashed into Åhlens Mall and burst into flames.
A second man being detained by Stockholm police
Armed police are currently at the scene and the incident is being treated as a suspected terror attack.
Stockholm metro stations have been shut following the attack.