The police deployment to Kos came after the island's mayor, Giorgos Kiritsis, warned of "bloodshed" if the situation on the island of 33,000 people -- where around 7,000 migrants are waiting to apply for immigration papers -- worsened.
"Two units, or 40 men, have arrived in Kos. Other reinforcements are being sent from other eastern Aegean islands," a police spokesman said.
European migration commissioner Dimitris Avramopoulos announced an emergency visit to Athens on Thursday for talks with Greek ministers as the leftist government grappled with the crisis.
The Greek government said it was "immediately" dispatching a ship to Kos to double up as an accommodation and processing centre for up to 2,500 people.
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"The ship will cover basic accommodation needs, identification will take place (onboard) and the difficult situation on the island will be to a large extent alleviated," Minister of State Alekos Flambouraris said in a statement.
Police on Kos on Tuesday had beaten back migrants with truncheons and sprayed them with fire extinguishers to prevent a stampede as the arrivals, mostly Afghans and Syrians, were being relocated to a local football stadium after camping along roads and beaches for weeks.
Meanwhile in waters off Libya about 60 migrants were missing, the International Organisation for Migration (IOM) said after interviewing survivors who were plucked out from their sinking rubber dinghy.
An Italian corvette, the Fenice, picked up dozens of migrants aboard the dinghy on Tuesday after the vessel was spotted by a naval helicopter.
Fifty-four were brought to the Italian island of Lampedusa, but according to testimony from survivors, the boat had set off with between 117 and 120 people on board, all of them from sub-Saharan Africa, the IOM said.