The Montenegrin government's decision on "economic citizenship" came into force on Tuesday, Montenegro-based website CdM is reporting.
An investor can get Montenegrin citizenship in several ways, including by making an EUR 100,000 payment to the account set up to finance "less developed local community unit."
Officials and politicians in Serbia on Sunday condemned Podgorica's decision to ban ceremonies marking the 100th anniversary of the unification of Serbia and Montenegro.
Serbian President Aleksandar Vucic said he doesn't understand the decision, which he said prohibits people in Montenegro from "thinking differently and having a different view of the past".
Both Croatia and Montenegro have said they are discussing the possibility of halting arms exports to Saudi Arabia in the wake of the death of dissident journalist Jamal Khashoggi, but neither has made a decision yet.
The OSCE South east Europe Media Conference, meeting in Struga, Macedonia, on Thursday adopted a declaration condemning the attack on Montenegrin Vijesti journalist Olivera Lakic, and called on the authorities in Montenegro to ensure that the investigation is independent, effective and speedy and that the perpetrators, including its masterminds, are brought to justice.