Fierce clashes between pro- and anti-regime protesters in Yemen's capital Sanaa left at least 25 injured on Thursday while troops deployed in force in the restive southern port city of Aden.

About 2,000 protesters, mostly students, had just left Sanaa University headed for Tahrir Square, or Liberation Square, when they ran into regime supporters and clashes broke out for a fifth straight day.

President Ali Abdullah Saleh's loyalists were seen gathered and waiting near the university since early in the day and attacked the protesters with batons and stones.

The protesters, chanting "The people want to overthrow the regime," responded by hurling stones.

Police intervened with warning shots to separate the two sides, but later withdrew, as the protesters came under fire from Saleh supporters, a journalist in Sanaa said. Fifteen of the injured were protesters and the rest were supporters of the regime, he said.

In Yemen's main southern city of Aden, the site of violent clashes between protesters and police that left two dead on Wednesday and prompted a heavy army deployment, at least 20 people have been arrested, a local official said.

Demonstrators on Wednesday had hurled stones at police, set tyres and vehicles on fire and stormed a municipal building where heavy gunfire rang out.

"Most of those arrested were held when they tried to storm into a police centre and the central prison in Al-Mansura (district) but they were stopped," the official told AFP.

Hundreds of protesters also broke into shops and three hotels, and set car tyres ablaze and blocked roads. "The police intervened only when the protests turned violent," he said.

Meanwhile, Saleh ordered the set up of an "investigative committee to inquire about the unfortunate riots that have occurred in some parts of" Al-Mansura, state news agency Saba reported on Thursday.

In Taez, south of Sanaa, protests continued for a sixth day as demonstrators carrying "Leave Ali" banners set up tents at a road intersection near Al-Huraish Square after police chased them away.

The protesters have called for a "Day of Rage" on Friday, with the authorities in Taez reacting with a heavy deployment of Republican Guard forces and the army.

In Ibb, southwest of Sanaa, hundreds of protesters took to the streets on Thursday demanding that Saleh step down.

On Tuesday, police in Sanaa stepped in when supporters and opponents of the president clashed, leaving three injured. The rival camps have likewise clashed in Taez.

On Monday, rocks and batons flew in the capital as protesters -- mainly students and lawyers -- confronted police and Saleh's supporters. Similar clashes broke out the previous day.

Anger at rampant corruption has helped to fuel unrest in Yemen, the poorest country in the Arab world.

In Sanaa, protests have becoming increasingly violent, despite Saleh -- elected to a seven-year-term in September 2006 -- urging dialogue on forming a government of national unity.

Separately, thousands of supporters of the secessionist Southern Movement staged protests in several southern cities calling for the secession of the formerly independent South of Yemen, witnesses said.

In the town of Daleh, hundreds took to the streets to mark "the prisoner's day" which falls on every Thursday. A similar demonstration took place in the town of Habilayn, which has been under tight army control since December.

Similar protests took place in towns of Loder in Abyan and Azan in Shabwa, calling for "disengagement."

Besides poverty and unemployment, Saleh's government is grappling a secessionist movement in the south, rebellion in the north, and a regrouping of Al-Qaeda on its soil.