Lebanese activists angered by a trash crisis and government incompetence stepped up their campaign Tuesday, occupying part of Lebanon's environment ministry to demand the minister's resignation.

The surprise move by the "You Stink" campaign came hours ahead of their deadline for the government to respond to a list of demands after a massive weekend demonstration.

The group of protesters sat cross-legged in a hallway outside Environment Minister Mohammed al-Mashnuq's office, waving Lebanese flags and chanting "Out, out out! Mashnuq, get out!"

"Peaceful, peaceful," the demonstrators called out.

More than four hours after the protesters entered the building, riot police had moved into the ministry in central Beirut and were gathered in its stairwell.

They expelled media broadcasting from inside the room where protesters were gathered and demanded they leave the building.

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: Mashnuq's resignation, new parliamentary elections, the devolvement of trash collection to municipalities and accountability for violence against protesters.

Tens of thousands of people converged on downtown Beirut on Saturday to protest a government they say is too corrupt and divided to provide citizens with basic services.

"They refused to listen to our demands that we gave them 72 hours to fulfil," activist Lucien Bourjeily told AFP.

"They announced clearly that the minister will not resign. We are asking for the resignation of the minister and won't leave until that happens."

- 'Element of surprise' -

Sit-ins in government buildings are rare in Lebanon, where public protests are typically called by political leaders unlike the "You Stink" campaign championed by civil society.

At the Beirut protest, organisers gave the government 72 hours to respond to their demands, with that deadline set to expire Tuesday night.

"(We began early) for the element of surprise," Bourjeily said of the sit-in.

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

Fellow organiser Marwan Maalouf pinpointed the blame.

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

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.

And according to Lebanon's National News Agency, Interior Minister Nuhad Mashnuq asked security forces to negotiate with those "occupying a public facility".

On its Facebook page, "You Stink" called on supporters "to join us at the ministry of environment... to demand the resignation of the minister for failing to solve the trash crisis."

An employee at the ministry, who spoke on condition of anonymity, confirmed that Mashnuq remained in his office, though most of the employees had left.

- 'Parliament of trash' -

Local television stations broadcast footage of a row of riot police equipped with helmets and shields stationed at the ministry.

Crowds of protesters were gathered outside the building, chanting up to the demonstrators inside who waved and called back.

"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.