Hundreds of Tunisians took to the streets of the capital and the coastal city of Sfax on Friday, waving yellow cards and demanding that the government reverse austerity measures.

More than 200 young people rallied in Tunis following a call from the Fech Nestannew (What Are We Waiting For?) campaign for a major protest against the measures imposed at the start of the year.

They held up yellow cards and chanted slogans amid a major deployment of riot police as they marched on administrative offices in the capital.

"The people want the Finance Act repealed" and "The people are fed up with the new Trabelsi", they shouted, referring to the graft-tainted in-laws of ousted president Zine El Abidine Ben Ali.

"We believe that dialogue and reforms are still possible," said Henda Chennaoui of the Fech Nestannew campaign.

"We've got the same demands we've been seeking for years -- to tackle real problems like the economic crisis and the high cost of living," she told AFP.

Friday's demonstrations came ahead of the seventh anniversary of Ben Ali's ouster on January 14, 2011.

In Sfax, some 200 kilometres (125 miles) south of Tunis, around 200 people vented their anger over rising prices, an AFP correspondent reported.

"The people's money is in the palaces, and the children of the people are in the prisons," read one placard.

Authorities said Friday the number of people detained in the wave of violent protests had risen to nearly 800, after a provincial town was hit by a night of unrest over the austerity measures.

An AFP correspondent in the northern town of Siliana said police fired tear gas at dozens of youths who pelted them with stones during skirmishes that lasted around three hours overnight.

Interior ministry spokesman Khalifa Chibani said 151 people were arrested Thursday, taking the number detained for alleged involvement in the violence to 778 after several nights of unrest.

- 'Heavy-handed methods' -

Chibani said clashes between youths and police were "limited" and "not serious", and insisted no acts of violence, theft or looting were recorded Thursday evening.

Friday's demonstrations were reported to be peaceful.

Rights group Amnesty International accused the authorities of using "increasingly heavy-handed methods to disperse rallies and subsequently arrest protesters" during the unrest.

"Tunisian security forces must refrain from using excessive force and end their use of intimidation tactics against peaceful demonstrators," the watchdog said.

The United Nations expressed concern at the number of arrests, and urged the authorities to ensure people can protest peacefully.

One man died in the unrest on Monday night, but the authorities have insisted the police were not responsible for this.

A number of left-wing activists have been arrested by the authorities after officials accused them of fuelling the violence.

Several dozen members of the Popular Front party demonstrated Friday in front of a court in the town of Gafsa after the arrest of several local activists, an AFP correspondent said.

Tunisia is considered a rare success story of the Arab Spring uprisings that began in the North African country in 2011 and spread across the region, toppling autocrats.

But the authorities have failed to resolve the issues of poverty and unemployment.

A spokesman for the prime minister in a statement said the protesters were "thugs aged between 17 and 21 who are not affected by the impact of the finance law".

Political scientist Hamza Meddeb said there was "very strong social anger" over a "political class increasingly cut off from the population" and because protests had not yet resulted in any concrete improvement.

Protests are common in Tunisia in January, when people mark the anniversary of the revolution that ousted long-time dictator Ben Ali.

The authorities on Friday said four people had been arrested after a petrol bomb attack on Wednesday damaged the entrances of two Talmudic schools in a Jewish district of Djerba.

The interior ministry said the aim of the perpetrators was to "sow chaos like that recorded in some parts of the country