ROME (AP) — Migrants in a Rome piazza threw rocks, bottles and gas cans at police in riot gear who were using water jets to clear out about 100 mostly Ethiopian and Eritrean asylum-seekers early Thursday as part of a security operation criticized by humanitarian groups.

Police say at least two people were detained, and 13 were injured.

The dawn piazza operation comes days after authorities cleared most out of the 800 migrants, primarily asylum-seekers, who had been squatting in a nearby building since 2013.

Police said in a statement the operation was necessary because the migrants remaining in the piazza had refused to accept city-organized lodging and because of the risk of cooking gas canisters and other flammable materials.

Authorities have said such raids to clear migrants from buildings and squares, at least four since July in Rome, are part of anti-terrorism measures.

But asylum-seekers, mostly women and children, who had been allowed to remain temporarily in a nearby building hung signs out of its windows saying "We are not terrorists." They were also cleared out.

The U.N. refugee agency, UNICEF and humanitarian organizations have protested Italy's moves, saying they were carried out without warning and that there is not enough adequate housing for them and the hundreds more vulnerable asylum-seekers sleeping on Rome's streets. They also noted that many families would be separated by the new housing arrangements and children would be uprooted from schools.

"It is a shame that the absence of alternative housing solutions brought about a violent situation," Doctors Without Borders said, calling for "dignified solutions" for those who have been removed.

On Twitter, Doctors Without Borders posted pictures of the police operation and protested the "indiscriminate use of violence." The organization noted there were no ambulances nearby to help, and said their volunteers treated 13 injured, mostly cuts and fractures, including an elderly woman who fainted after being hit by a water jet.

Italy is struggling to meet the demand to house migrants, with more than 98,000 arriving so far this year after being rescued at sea in rickety smugglers' boats. More stringent border controls are preventing most of those migrants from continuing their journeys to preferred destinations further north.

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Colleen Barry reported from Milan.