An adviser to Syrian President Bashar al-Assad has promised government forces will not fire on protests planned for Friday, an opposition leader says.
Louay al-Husein told the BBC he and other opposition leaders had been in talks with adviser Buthaina Shabaan to negotiate an end to the crisis.
Reports say tanks and soldiers are already gathering in some cities.
Thousands of people may have been detained in Syria since protests began in March, the Red Cross says.
Mr Assad's government insists it is pursuing "armed terrorist gangs".
Mr Husein, who was detained at the start of the unrest but freed a few days later, said Ms Shabaan had told the opposition that security forces had been given strict orders not to fire on crowds on Friday.
Ms Shabaan also said talks to end the crisis would continue next week, Mr Husein added.
Some reports suggested security forces were being deployed in cities where protests are expected on Friday.
In the central city of Homs, a resident told AP news agency that soldiers had set up sand barriers with machine guns perched on top, and said that three tanks were still in the area.
"It seems they are getting ready," he told AP.
Syrian soldiers and tanks also surrounded the city of Hama, the news agency reported.
The International Committee of the Red Cross says it fears hundreds and perhaps thousands of people in Syria have been detained by security forces during the recent unrest.
Beatrice Roggo, the Red Cross head of operations in the Middle East and North Africa, told the BBC that lack of information about the exact number of detainees held by the authorities was a great concern.
Ms Roggo also appealed to the Syrian government to allow the Red Cross access to all parts of the country where people were in need.
Meanwhile, government forces continue to arrest democracy campaigners in several cities, the Syrian Observatory for Human Rights (SOHR) said.
The SOHR said security forces arrested dozens of people on Thursday in Banias and the neighbouring villages of al-Bayda and al-Qariri.
Lawyer Jalal Kindo was among those detained in Baniyas, where security forces have been searching out protest organisers, the London-based group said.
Prominent rights campaigner Najati Tayara was meanwhile arrested in Homs, the Syrian Centre for the Defence of Prisoners of Conscience said.
On Wednesday, 18 people were reportedly killed as tanks shelled Homs and clashes were reported in towns and villages around Deraa, where the protests began.
Security forces also broke up a rare demonstration in Aleppo, Syria's second largest city, involving thousands of students, witnesses said.
The SOHR says 647 civilians have been killed since pro-democracy protests began on 18 March. Another rights group, Sawasiah, says more than 800 civilians have died.
Officials dispute the civilian toll and say about 100 soldiers have died.
A mobile phone snapshot, reportedly taken in Qamishli on 29 April, shows protesters carrying banners written in Arabic and Kurdish demanding democracy.
Razan, who is a resident of Damascus, tells the BBC about violence and protests around Syria.
This unverified video seems to show a peaceful protest in Talbisah. Moments into the footage, tanks fire on unarmed civilians. Wyre Davis reports.
Residents of Deraa walk past a burnt-out building. It follows shelling by troops in what human rights groups say was an intensified crackdown on protests in recent weeks.
Syrian army vehicles were photographed near Homs and broadcast on the Syrian opposition internet channel Sham SNN on 11 May.