The men were able to drive away, despite police pursuing them.
The boy was out for a walk with his nanny at 10:30am when two dark grey vans with Hungarian number plates drew up beside them. Two men jumped out and grabbed the boy.
Police believe his mother and grandfather were in one of the vans. Police stopped one of the vehicles on the A4 but there was no trace of the boy, or his mother.
When the boy’s Austrian father, 46, and his Hungarian partner split up in 2013, they received joint custody. However during a visit to Hungary the boy’s mother announced that she and her son would not be returning to Austria.
"The father then sought sole custody," the father's lawyer Britta Schönhart said.
In April 2014 he took his son back to Vienna. At the end of July a court in Vienna ruled that it was better for the boy to stay in Austria. However, it seems like his mother may now have resorted to vigilante justice.
In another international custody case Austrian Marion Weilharter recently appealed to the Austrian foreign minister to help her get her son Oliver back from Denmark, where his father took him.
A court in Graz convicted Thomas Sørensen to a 12-month suspended sentence for kidnapping his son and taking him to Denmark in 2012.
However Denmark granted Sørensen custody of Oliver, and refuses to return him to Austria.