"There is an arrest campaign and phone lines are down so people are suspecting interference but the government says it is a technical problem," a resident of Douma, north of Damascus, told AFP.
In the capital, the internet was down and cell phone communications were practically impossible, although land lines seemed to be working fine.
A customer representative of Syria's phone and internet provider told AFP the problem "was due to an overload of connections" without adding further details.
Syriatel and MTN, Syria's two phone companies, offered customers one hour of free calls valid between April 2 and 6 "in recognition of the people who stood with the President (Bashar) al-Assad during the day of dignity."
They were referring to pro-regime demonstrations in the capital Tuesday.
But on Friday, the Muslim day of rest, thousands of Syrians marched across the country, calling for reforms, disappointed by a presidential speech which failed to lift a state of emergency in place since 1963.
In a joint statement, eight human rights groups said 46 people were arrested in raids that targeted the southern town of Daraa, one of thee main centres of more than two weeks of demonstrations, as well as Douma north of Damascus and the industrial city of Homs.
"We condemn this extremely violent and unjustified way the Syrian security services dealt with peaceful rallies in Douma where police used excessive force against demonstrators," said the statement.
The rights groups reported that four people died and dozens were wounded in the crackdown. A human rights activist reported eight dead.
A witness told AFP security forces used live ammunition to disperse stone-throwing protesters after noon prayers.
The authorities denied the security forces were responsible for the deaths, blaming them on an "armed group" which opened fire from rooftops on both demonstrators and police.
They acknowledged an unspecified number of deaths and said dozens were wounded, some of them policemen.
State television charged that "some of the demonstrators had daubed their clothes with red dye to make foreign reporters believe that they had been injured."
Some 200 people demonstrated outside the courthouse in Daraa, a tribal town near the Jordan border, where security forces arrested eight people between a morning raid and a round-up after the protests, an activist told AFP.
Security forces carried out a series of raids in the area, another activist said, adding that architect Khaled al-Hassan, lawyer Hassan al-Aswad and teacher Issam Mahameed were among those detained.
Yusef Abu Rumiyeh, a member of parliament for Daraa, denounced security forces for opening fire on his constituents "without pity" and criticised Assad for not offering his condolences.
The security forces "opened fire on the citizens of Daraa, killing and injuring them and preventing the wounded from getting to hospital," said Rumiyeh, in a video uploaded to YouTube.
"The people of Hauran were waiting for President Assad to visit to offer his condolences. Had he done so, nothing that happened subsequently would have taken place."
In Homs, the Syrian Observatory for Human Rights documented 17 arrests while authorities accused another "armed group" of firing on demonstrators in the industrial city and killing "one girl."
The rights group demanded the release of all prisoners of conscience and political prisoners, and called for measures to ensure the safety of peaceful protest.