REUTERS
The Greek government has asked ferry operators to reduce services bringing migrants from islands to the capital Athens in a bid to ease pressure, according to the BBC.
It is trying to slow the flow of migrants to its northern border, to prevent a further build-up of people trying to reach other EU states, the BBC wrote.
Countries further north have imposed border restrictions that make it harder for arrivals to move on from Greece.
About 2,800 people massed on Greece's border with Macedonia on Thursday.
Only 100 were allowed to cross, correspondents say.
Around 2,000 migrants arrived at the port of Piraeus, near Athens, on Thursday alone.
However, many cannot now travel further north, and the numbers stuck in Greece are building.
In northern Greece, protesters blocked the entrance to a new proposed camp for migrants near Polykastro.
Some local people said they were concerned that the camp would become permanent.
Greece had already recalled its ambassador to Austria on Thursday amid sharp divisions among EU states over the migrant crisis.
It acted after Austria hosted a meeting on the migrant issue with Balkan states, to which Greece was not invited.
The Athens government has been criticized by other EU countries for failing to manage the new arrivals - but no progress has been made on a European plan to relocate refugees from Greece.
Prime Minister Alexis Tsipras has threatened to block all decisions at EU migration summits next month if member states do not agree to take in quotas of migrants.