As many as 28 protesters have been killed across Syria after security forces reportedly shot at protesters, hundreds of thousands of whom took to the streets in the biggest protests so far against Bashar al-Assad's rule, an activist group said.

The latest toll included 22 people who were killed in Damascus and its suburbs - the highest death toll for the capital so far, Mohammad Abdullah, a spokesman for the Local Co-ordination Committee (LCC), said on Saturday.

The LCC tracks anti-government demonstrations in the strife-ridden country.

Police fired live ammunition and teargas in Damascus, killing five people, and four in southern Syria near the Jordanian border, Reuters news agency reported on Friday, quoting witnessess and activists.

Three protesters were shot dead in the northern city of Idlib, they said.

Reuters quoted a witness in the Rukn al-Din district of Damascus as saying that hundreds of young men wearing white masks fought security forces with sticks and stones.

"Down, down Bashar al-Assad", they chanted.

"We are in Midan and they are firing teargas on us, people are chanting," a witness said by telephone from the centre of Damascus.

In the city of Hama, scene of a 1982 massacre by the military, live video footage by residents showed a huge crowd in the main Orontos Square shouting "the people want the overthrow of the regime".

Al Jazeera has not been able to verify independently the reports of violence.

Deir al-Zour unrest

At least 350,000 people demonstrated in the eastern province of Deir al-Zour, a tribal city in the eastern desert, the Syrian Observatory for Human Rights (SOHR), an independent rights group based in London, said.

"These are the biggest demonstrations so far. It is a clear challenge to the authorities, especially when we see all these numbers coming out from Damascus for the first time," Rami Abdelrahman, head of the SOHR, said.

On Thursday day, Syrian forces shot dead two pro-democracy protesters in Deir al-Zour, local residents said.

For the first time, that city observed a full strike on Thursday, an activist told Al Jazeera, with almost all businesses closed, though government offices remained operational.

Reports said military dragnets had also taken place on Wednesday and Thursday in Damascus, Idlib province and a politically sensitive area near the Turkish border in the northwest.

President Assad, facing the greatest challenge to 40 years of Baath Party rule, has sought to crush demonstrations.

The protesters have been calling for reforms and an end to the longstanding political status quo.

Swelling in size

Rights groups say about 1,400 civilians have been killed since the uprising began in March, but the protests have continued unabated and swelled in size.

With the economy stagnating and unemployment rising, Syria's main ally, Iran, is considering offering $5.8bn in financial help, including a three-month loan worth $1.5bn to be made available immediately, a French business newspaper, Les Echos, said, citing a report by a Tehran think-tank linked to Iran's leadership.

Emboldened by the spreading protests, prominent opposition figures and activists were to hold a conference in Istanbul in Turkey on Saturday that would be closely co-ordinated with another conference in Damascus.

Radwan Ziadeh, an opposition figure, told Reuters the conference would elect a 75-member National Council consisting of opposition members from inside and outside Syria.

"This is the first time we have a joint dialogue [between] opposition in Syria and exiled Syrians," he said from Istanbul.

"We will elect 50 members to the National Council from inside Syria and 25 from exiled Syrians."