A long queue began forming at 7.00 am outside the Dutch capital's landmark Carre Theatre, with those in line saying they want to pay their final respects to a Dutch "hero".
A prominent reporter who had been involved in a court case against one of the country's most wanted drug barons, De Vries, 64, was shot at least five times as he left a television studio on July 6.
He died in hospital nine days later.
"It's important to say goodbye," said a somber-looking mourner Brenda van Coevorden, 66.
"I am so sad that we've lost an icon," she told AFP.
"He meant a lot to the Netherlands. He's done so many good things," added Ruth van Rosmalin.
He was our hero. He stood up for people who couldn't do it themselves," she said.
Inside the theatre a white coffin lay draped with dozens of red roses could be seen, with a large photograph of De Vries at the top.
An Ajax football jersey bearing De Vries' name -- the late journalist was a massive fan -- stood in a frame next to the coffin.
Mourners were asked not to take too long in paying respects as thousands of people were still expected to visit the coffin throughout the day.
Many of those leaving the theatre were in tears, an AFP correspondent said.
Family and friends of De Vries are to say their farewells on Thursday during a closed ceremony, while radio stations are to play De Vries' favourite song throughout the day, Dutch national newscaster NOS said.
Two men were arrested shortly after the shooting and remain in custody while a police investigation continued.
The attack on De Vries, who first won fame for his inside reporting on the 1983 kidnapping of Heineken millionaire Freddy Heineken, sparked widespread condemnation and concern for the safety of journalists in Europe.
De Vries often appeared as a commentator or spokesman for families of crime victims, particularly in so-called "cold cases".
Most recently he acted as advisor and confidant of Nabil B., the state's key witness in the case against Ridouan Taghi, described as the country's most wanted criminal.