- NEW: Their assets are frozen, and they're subject to a travel and arms ban
- The European Union accuses the officials of "violence against demonstrators"
- EU: Maher al-Assad, the president's youngest brother, is the "principal overseer" of the crackdown
- Syrians have been demonstrating against the government as the "Arab Spring" sweeps the region
(CNN) -- The European Union slapped sanctions on 13 top Syrian officials, including President Bashar al-Assad's brother, for "violence against demonstrators," it announced Tuesday.
Maher al-Assad, the president's youngest brother, is commander of the army's 4th Division and "principal overseer" of the crackdown against protesters, the EU said.
The head of Syrian intelligence, Ali Mamlouk, and Interior Minister Mohammad Ibrahim al-Chaar, were also sanctioned, along with 10 other intelligence, security and business officials.
Their assets in Europe are being frozen, and they're now subject to an arms embargo and travel ban.
Syria has been cracking down forcefully for weeks on anti-government protesters inspired by the "Arab Spring" sweeping across the region.
Protesters have demanded the immediate release of political prisoners, lifting of emergency and martial law, and withdrawal of intelligence forces from Syrian cities.
Last month, President al-Assad lifted the country's 48-year-old state of emergency and abolished the state security court, both of which were key demands of the demonstrators. But anti-government protests have continued, with activists alleging massive human rights abuses and calling for political and economic reforms.
Security forces have relied heavily on the army and the regime's guard to disperse demonstrations.
Syrian security forces were using soccer stadiums as makeshift prisons Monday in at least two cities -- Banias and Daraa, the center of the uprising -- after raiding homes and arresting hundreds of residents, the directors of two human rights organizations said.
In addition, security forces burst into homes and took residents into custody in the Damascus suburb of Modemiyah, where there were reports of gunfire Monday, said Rami Abdul-Rahman, director of the Syrian Observatory for Human Rights.
And in the city of Homs, a doctor said tanks were dispersed and security checkpoints were set up at the entrance to each suburb, with security forces, often backed by secret police, searching anyone passing through. The security presence was so heavy that people were afraid to demonstrate, the resident said.
The witness in Homs said he knew several people killed in demonstrations Friday who could not be buried, because security forces were not allowing funerals out of fear they may turn into demonstrations.
The doctor said he has seen several public buildings, including two health centers and a school, transformed into command and control centers for the Syrian security forces and military. Dozens of people have "disappeared," and their families believe they have been arbitrarily detained, the doctor said.
A U.N. humanitarian assessment team due to enter Syria was stopped despite having previously been given permission by authorities, a U.N. spokesman said Monday.
The mission was set to visit the city of Daraa, site of a crackdown by government forces.
U.N. spokesman Farhan Haq said Monday the United Nations was trying to clarify why it wasn't able to enter.
More than 400 people have been arrested in Banias since Saturday, Abdul-Rahman said, adding that authorities had converted the Mediterranean city's soccer stadium into a prison to house them.
In the southern city of Daraa, the hub of Syria's six-week uprising, another human rights organization observed a similar situation.
"In Daraa, there have been so many arbitrary arrests in recent days that the army and security forces are using schools and the city's soccer stadium as makeshift prison facilities," said Ammar Qurabi, chairman of the National Organization for Human Rights in Syria.
The heads of both organizations are not in Syria but remain in close contact with activists and protesters there.
CNN has not been granted access into Syria and is unable to independently verify witness accounts.
On Monday, headline on the state-run news service, SANA, stated that participants in the "public and legal campaign to prosecute sedition promoters" were set to prosecute the "conspirators" and that 1,083 people "involved in riot acts" had turned themselves in.
The "conspiracy against Syria aims at fragmenting Syria's national unity and destabilizing it," one of the articles read.
Human rights organizations criticize the arrests and violence, calling on Syria's government to establish an independent commission to investigate.
"We condemn the continued use of the Syrian authorities of violence and excessive force against the citizens of Syria demonstrating peacefully," said a statement from six organizations posted on the National Organization for Human Rights in Syria's website Monday.
The statement also expressed "deep concern" for those arrested.
"We call on security forces to stop the arbitrary arrests that take place outside the law and which constitute a flagrant violation of fundamental rights and freedoms guaranteed by the Syrian Constitution of 1973," the statement said.
Amnesty International said Friday that more than 540 people have been killed by Syrian security forces and that many activists in pro-reform protests "have been forced into hiding after receiving threats from Syrian authorities."