Travel advisory: Travelers are advised to wear full-face masks, avoid eating in crowded areas and be prepared for crowds of thousands, the Greek Ministry of Health said Monday.
The advisory comes after a string of recent attacks on tourists and other tourists that left at least five dead and several injured.
In recent days, Greek authorities have arrested more than 30 people on suspicion of involvement in the assaults and said they would launch a full investigation.
The government says the attacks are linked to anti-government demonstrations, and are also the work of foreign gangs.
They have targeted tourists and foreigners, as well as foreigners in Greece.
The ministry says the Greek government has also urged tourists to wear masks, take extra care in public, and not to go outside at night.
The attacks began at about 6:30 p.m.
(1600 GMT) Saturday when two gunmen opened fire at a cafe near the historic Acropolis Palace.
They also stormed the nearby Golden Dawn party and an amphitheatre.
The attackers killed three men, including a Greek tourist.
They also injured two other men and wounded two other foreigners.
Two of the victims were later killed by police.
Another four were injured.
Police found two AK-47s and four other weapons in the apartment.
The Golden Dawn and its party are under investigation for possible links to the attacks.