Sudan's opposition parties have pledged to step up anti-regime protests, their alliance said on Thursday, nearly three weeks after the start of unprecedented public demonstrations sparked by inflation.

About 17 political parties on Wednesday signed a charter "to dismantle the one-party regime and bring about a multi-party system," Farouk Abu Issa, head of the opposition coalition, said.

The change will come "by mobilising our people" with larger peaceful protests against the ruling National Congress Party (NCP), including on Friday which has become a focus of anti-government demonstrations, he said.

"This is the way we have chosen," Abu Issa added.

Non-governmental organisations and individuals also signed the pact.

Protests against high food prices began on June 16 at the University of Khartoum.

After President Omar al-Bashir announced austerity measures, including tax hikes and an end to cheap fuel, the protests spread to include a cross-section of people around the capital and in other parts of Sudan.

Demonstrators have burned tyres, thrown stones and blocked roads in a call for regime change that has been met by what the European Union called "a violent crackdown."

Last Friday, hundreds of protesters who gathered beside the Umma party mosque in Khartoum's twin city of Omdurman were confronted with tear gas and rubber bullets, witnesses said.

The Umma party, led by former prime minister Sadiq al-Mahdi, is a key member of the opposition alliance, along with Islamist opposition leader Hassan al-Turabi and his Popular Congress Party.

A diplomatic source last week said that opposition parties had been relatively quiet during the protests, and activists have dismissed the political opposition as weak.

But Wednesday's pact could lead to coordination, which has been missing, between those who have been protesting on the street and the political opposition, one activist said.

"Now hopefully this agreement that they reached yesterday will allow them to make a call... for a national protest," said the activist, who was previously arrested and asked for anonymity.

"If they do that, that will have a very significant impact because they do have numbers. The Umma party can rally 50,000 people in a day."

There will be "a huge impact" if imams such as Mahdi ask people to take to the streets in protest after Friday prayers, the activist added.

The NCP's political secretary, Hasabo Mohammed Abdel Rahman, on Thursday said his party remained committed to dialogue "with all political forces," the official SUNA news agency said.

He added that numerous parties were participants in the current government, and that those who opposed the austerity measures had presented no economic alternative "but managed to stage protests and demonstrations."

Abu Issa noted that popular mobilisation has twice toppled regimes in Sudanese history.

In 1964, the death of student activist Ahmed al-Qureshi sparked the "October Revolution" which brought down the military government then in power after tens of thousands protested.

During an economic crisis in 1985, huge crowds marched in an uprising which led to the bloodless overthrow of president Gaafar al-Nimeiry.

On June 30, 1989, Bashir seized power from Mahdi, who was democratically elected.

Under the current regime the civil service, military and "all governmental institutions have been hijacked by the National Congress Party," Abu Issa said.

He vowed to form a coalition government that respects international norms of civil liberties, has "brotherly" relations with South Sudan and responds to the demands of people in the Darfur region.

Abu Issa said he would not exclude the NCP if they wanted to join as "partners".

Bashir has played down the current demonstrations as small-scale and not comparable to the Arab Spring uprisings in Egypt and elsewhere, maintaining that he himself remains popular.