Lebanese police moved to eject protesters who occupied part of the environment ministry on Tuesday, in an escalation of a campaign against the country's trash crisis and a stagnant political class.

The forcible evacuation began some six hours after several dozen protesters entered the ministry to demand the resignation of Environment Minister Mohamed Mashnuq.

Activists from the "You Stink" campaign said police had beaten protesters, and a Red Cross official said medics treated several people for wounds sustained in scuffles with security forces.

The sit-in was an unexpected escalation of a campaign that began over a trash crisis but has evolved into a broad-based movement against government impotence and corruption.

As night fell, several hundred protesters demonstrated outside the ministry in support of the last activists who remained inside, occasionally skirmishing with security forces.

"We will continue! Revolution!" they chanted.

A security source confirmed that the evacuation of protesters had begun, with police removing a group gradually from the seventh floor of the building in central Beirut.

A source at the interior ministry said some 14 demonstrators remained inside and were insisting police would have to handcuff and remove them by force.

The "You Stink" campaign said several activists had been beaten by police.

"(Activist) Lucien Bourjeily and a group of people participating in the sit-in were beaten and cannot be reached," the campaign said on its Facebook page.

A Red Cross official said 14 people were treated at the scene for light wounds sustained in confrontations with police.

A 15th person was treated for respiratory problems, and one activist was taken to hospital for treatment, the official said.

The "You Stink" campaign began in response to a crisis that erupted with the closure of Lebanon's largest landfill in mid-July.

But it has evolved into an outlet for deep-seated frustrations over Lebanon's crumbling infrastructure and stagnant, confessional political system.

Last week, the campaign set out four key demands and set a deadline, which coincided with the start of the police evacuation: Mashnuq's resignation, new parliamentary elections, the devolution of trash collection to municipalities and accountability for violence against protesters.

On Saturday, the campaign scored its biggest turnout yet, with tens of thousands of people converging on a square in downtown Beirut for a virtually unprecedented protest.

At the end of that demonstration, the activists gave the government 72 hours to respond to their demands, though they began their sit-in in the ministry several hours before the deadline expired.

"(We began early) for the element of surprise," activist Bourjeily told AFP as the sit-in began.

He said additional measures could be announced later Tuesday. "It will depend on the response of the government."

Fellow organiser Marwan Maalouf laid the blame directly on Mashnuq, who has been criticised for failing to draw up a solution to the trash crisis.

"The trash is piling up in homes everywhere, and who is responsible for this situation? It's the minister of environment," Maalouf said.

The closure of the country's landfill was announced last year, but despite the lead time, the government found no alternative.

As an interim solution, municipalities have been dumping trash in empty lots, river beds and even valleys.

Mashnuq has said he will not resign as minister, but he withdrew on Monday from the cabinet committee dealing with the trash crisis.

A source close to Prime Minister Tammam Salam called on demonstrators "to wait for two or three days" to hear the proposals of an ad hoc committee set up on Monday and headed by Culture Minister Akram Chehayeb.

Earlier, "You Stink" urged supporters "to join us at the ministry of environment... to demand the resignation of the minister for failing to solve the trash crisis."

Throughout the sit-in, a crowd of supporters demonstrated outside the ministry.

"We don't want the parliament of trash, or the parliament of extension," they sang, referring to the Lebanese legislature which has extended its own term twice since 2009.

Parliament has been unable to elect a president since the post was vacated in May 2014, and deep political rifts have left the cabinet too divided to function.

Parliament speaker Nabih Berri has called for a dialogue among various political leaders to begin September 9, but activists called the plan a bid to divert attention from their demands.