KHARTOUM (Reuters) - Security forces rounded up some 30 people on Thursday as they dispersed protesters close to Sudan’s presidential palace calling on President Omar al-Bashir to quit, witnesses said.

At their first joint news conference on Wednesday, Sudan’s main opposition groups also issued a call for Bashir to step down, saying they have agreed on a “programme that would take place after the regime falls”.

The government in turn threatened action against groups it accused of advocating violence.

The protests, held near daily since Dec. 19 over frustrations with economic hardships, have become the most sustained challenge to Bashir’s three decades in power.

In their latest protest in downtown Khartoum on Thursday, demonstrators chanted “Peaceful, peaceful against the thieves” and “Down, that’s it!”, witnesses said.

They said security forces in plainclothes and armed with plastic pipes rounded up protesters close to the presidential palace and drove them away on pickup trucks.

Police also chased other activists through side streets as smaller rallies broke out.

A police spokesman could not be reached for comment.

The president and his ruling National Congress Party have shown no sign of bowing to those demands and have blamed the unrest on unnamed foreign powers. He and senior officials have used more conciliatory language in recent weeks, promising to release detained demonstrators.

But activists say hundreds remain in detention.

An opposition-linked doctors’ syndicate said last week that 57 people have been killed in the protests. The government puts the death toll at 31, including two security personnel.

Security forces have regularly used teargas, stun grenades and live ammunition to break up demonstrations.

The unrest has been fuelled by a deepening economic crisis marked by high inflation and shortages of bread, petrol and cash. The Sudanese pound fell to a record low on the black market on Thursday.

Speaking to journalists on Wednesday, the opposition said that should Bashir go, they had agreed to hold a constitutional dialogue conference at the end of a four-year transitional governance followed by elections.

The Information Ministry condemned the opposition parties, saying they had rejected dialogue and opted instead for violence to bring down the regime.

“The government will take the necessary legal measures to respond to calls for violence and political and ideological terrorism and change by force,” it said in a statement.

Bashir came to power in 1989 after he overthrew the elected government of Sadiq al-Mahdi, and had since won successive elections in a vote the opposition says are neither fair nor free.