At least 20 refugee homes were demolished by Lebanese military units on Monday with more demolitions expected, international aid agencies said.
The government had given refugees in the village of Arsal, in the north-east of the country, until July 1 to demolish "illegal" shelters – those made of materials other than timber and plastic sheeting.
The small concrete huts that many Syrians live in break the rules of the informal camps, with some Lebanese fearing they would lead to lasting settlement in the country.
"At 4.30am on July 1, military units moved into several camps in Arsal and demolished at least 20 homes," said a joint statement by seven aid groups, including Save the Children, the Norwegian Refugee Council and Oxfam.
"We fear that this is the start and more demolitions will take place tomorrow."
The army did not comment on the claims.
Lebanon, a country of about four million people, hosts between 1.5 and two million Syrian refugees, but only a million of these are registered with the United Nations High Commission for Refugees.
Some Lebanese politicians have called for the mass return of refugees to Syria as fighting in the eight-year-long civil war has abated in many parts of the country.
Last week, The National reported on Lebanon's crackdown on the hiring of Syrians as public opinion is increasingly turning against the refugee population.
