BEIRUT -- The Syrian army sent more tanks and reinforcements into Daraa on Wednesday as part of a widening crackdown against opponents of President Bashar Assad's authoritarian regime, and gunfire and sporadic explosions were heard in the tense southern city.
In addition to the unrest in Daraa -- where the uprising began more than five weeks ago -- security forces conducted sweeping arrests and raids elsewhere in the country, residents said. Witnesses and human rights activists also said the army deployed tanks around the Damascus suburb of Douma and the coastal city of Banias, the site of large demonstrations recently.
The crackdown came as opposition figures said their "massive grassroots revolution" will break the regime unless Assad leads a transition to democracy.
The statement from an umbrella group of opposition activists in Syria and abroad called the National Initiative for Change said a democratic transition will "safeguard the nation from falling into a period of violence, chaos and civil war."
"If the Syrian president does not wish to be recorded in history as a leader of this transition period, there is no alternative left for Syrians except to move forward along the same path as did the Tunisians, Egyptians and Libyans before them," the statement said.
The relentless government throttling of the protest movement showed no signed of letting up.
A vigorous crackdown since mid-March has killed more than 400 people across Syria, with 120 dead just over the weekend. The repression, however, has only emboldened protesters who started their revolt with calls for modest reforms but are now increasingly demanding Assad's downfall.
The head of the Syrian Observatory for Human Rights, Rami Abdul-Rahman, said the number of civilians killed since the uprising began reached 451. More than 50 soldiers also have been killed, he added.
The international community has stepped up its criticism of the crackdown by the Assad regime, with leaders raising the possibility of sanctions. German government spokesman Steffen Seibert said Wednesday that Berlin would strongly support European Union sanctions against the Syrian leadership.
In Paris, the Foreign Ministry said the European nations of France, Britain, Germany, Italy and Spain have summoned Syrian ambassadors across the continent to condemn the violence against protesters.
Human rights activist Mustafa Osso, who cited witnesses in Daraa, said new troops arrived early Wednesday from nearby military bases. Another rights activist, Ammar Qurabi, said security and armed forces have detained more than 400 people in Daraa since a crackdown in the city Monday.
The Syrian armed forces said in a statement that army units continued their operations in Daraa and the countryside to chase "extremist terrorist groups." It said the groups attacked troops near the Golan Heights, killing three officers and wounding 15.
Assad has blamed most of the unrest on a "foreign conspiracy" and armed thugs, not true reform seekers.
The opposition is getting more organized as the uprising gains momentum, but it is still largely a grassroots movement. There are no credible opposition leaders who have risen to the level of being considered as a possible successor to Assad.
The army sent tanks into Daraa, 80 miles (130 kilometers) south of Damascus, on Monday and there have been reports of shooting and raids there and elsewhere in the country ever since.
A video posted on a Syrian opposition page on Facebook showed a convoy of about two dozen flatbed trucks carrying tanks heading south. It said the video showed reinforcements being sent to Daraa on the highway that links the city with the capital, Damascus.
Witnesses and human rights activists said the army also deployed tanks around the Damascus suburb of Douma and the coastal city of Banias, where there have been large demonstrations in recent weeks.
One Douma resident said security agents were going house to house, carrying lists of wanted people and conducting raids. If the agents did not find the person they were looking for, they seized his relatives , the resident said.
Two funerals were planned Wednesday, he said.
A witness in Banias said the army redeployed tanks and armored personnel carriers near the main highway leading into the city.
In the coastal city of Latakia, an activist said security forces fired live bullets and a stun grenade at demonstrators in poor areas near the city's Palestinian refugee camp of al-Ramel. He said four people were wounded and several others were detained.
Residents contacted by The Associated Press spoke on condition of anonymity out of fear for their safety.
Syria has banned nearly all foreign media and restricted access to trouble spots since the uprising began, making it almost impossible to verify the dramatic events shaking one of the most authoritarian regimes in the Arab world.
In Turkey, activists held a two-day conference attended by Syrian opposition figures who called on authorities to stop the "bloodshed and random arrests." They said Syria is at a crossroads between freedom and prosperity or it could remain "a symbol for injustice and oppression."
U.N. Secretary-General Ban Ki-moon on Tuesday condemned the "continuing violence against peaceful demonstrators, most particularly the use of tanks and live fire that have killed and injured hundreds of people."
The European countries of France, Britain, Germany and Portugal circulated a draft media statement to the U.N. Security Council that would strongly condemn the violence against peaceful demonstrators, but the council put off a discussion of the draft until later Wednesday because some members didn't have instructions from their capitals.
Susan Rice, the U.S. ambassador to the U.N., said the United States has evidence of active Iranian support for the Syrian government's crackdown.
Syria's U.N. Ambassador Bashar Ja'afari said some armed groups have taken advantage of the demonstrations and started shooting. He said Assad told security forces not to open fire in response and claimed "dozens and dozens of security officers" were shot and killed.
The U.N.'s top human rights body agreed Wednesday to hold a special human rights session on Syria to try to stop Assad's forces from gunning down his people. The meeting will be held Friday.
Also Wednesday, Iraqi Deputy Migration Minister Salam al-Khfaji told the AP that authorities are expecting a "huge influx" of Iraqis returning from Syria because of the violence. Hundreds of thousands of Iraqis have been living in Syria since the 2003 U.S. invasion.