On Monday, Swedish Prime Minister Stefan Lofven, most of the royal family and Stockholm Mayor Karin Wanngard stood outside Stockholm’s City Hall for the ceremony.
Other tributes were paid across Sweden.
Police have not released the identities of the victims, but said they were:
- Two Swedish nationals – one reported to be an 11-year-old girl
- A Briton – named by his family as 41-year-old Chris Bevington
- A Belgian woman, said to be a 31-year-old from the city of Halle
Police are still questioning the main suspect, who has been named as Rakhmat Akilov by the Swedish media.
The suspect then disappeared, and police began searching for him, Mr Hysing added.
He was known to have expressed sympathy for groups including so-called Islamic State (IS), but had been seen only as a “marginal character”, police said.
Meanwhile, a second suspect has been placed under formal arrest.
No terrorist group has claimed to be behind the attack.
Sweden has taken in nearly 200,000 refugees and migrants in recent years – more per capita than any other European country.
However, there was a drop in numbers last year after the country introduced new border checks.
Separately, Sweden is believed to have the highest number of IS fighters per capita in Europe.
About 140 of the 300 who went to Syria and Iraq have since returned, leaving the authorities to grapple with how best to reintegrate them into society.