The Obama administration faced renewed pressure Thursday to toughen its stance on Syria, where pro-democracy activists remained defiant ahead of planned nationwide rallies to denounce government brutality against demonstrators.

Three key senators formally called on the White House to break publicly with Syrian President Bashar al-Assad and to impose sanctions against members of his government. The call for tougher measures came as European Union diplomats gathered in Paris to discuss similar steps against the Assad regime.

In a letter to President Obama, Sens. John McCain (R-Ariz.), Lindsey O. Graham (R-S.C.) and Joseph I. Lieberman (I-Conn.) demanded tangible steps to pressure Assad, whose security forces have killed more than 400 people in the past month, according to estimates by human rights groups.

“The escalating crackdown by Bashar al-Assad’s regime against the Syrian people has reached a decisive point,” the senators warned in the letter. “By following the path of Moammar Gaddafi and deploying military forces to crush peaceful demonstrations, Assad and those loyal to him have lost the legitimacy to remain in power in Syria.”

Administration officials said this week that they are considering new sanctions as part of a list of possible measures to dissuade Assad from using violence against peaceful protesters. Syrian forces are thought to have killed more than 100 civilians in a series of assaults on demonstrators last Friday, while dispatching tanks to the restive city of Daraa.

Syrian activists were bracing for a key test of their movement after the week-old crackdown by the government.

Messages posted to a Facebook page used by Syrian demonstrators called for people to turn out in large numbers across the country after Friday prayers to protest the recent killings in Daraa, which remained under siege by the Syrian army for a fourth straight day. At least 38 people have been killed there since Syrian tanks rolled into the town on Monday, shooting indiscriminately and firing artillery, according to human rights groups. Since then, water, electricity and communications have been cut off, and residents have described bodies piling up in the streets because citizens are too afraid to retrieve them.

Syria’s banned Muslim Brotherhood also called on Syrians to take to the streets to demand freedom. “Do not let the regime besiege your compatriots. Chant with one voice for freedom and dignity,” said its declaration, which was sent Thursday to the Reuters news agency.

In the Damascus suburb of Madaya, 87 people were detained and two were shot dead after security forces swarmed into the area shortly before dawn Thursday and went house to house rounding people up, according to Wissam Tarif of the human rights group Insan, which is monitoring the Syrian unrest. Three people were shot dead in the northern coastal city of Latakia in a late-night protest there, and there were also reports that several tanks had moved into the city, perhaps to deter further protests Friday.

“Friday will give us an indication of what is winning: Is it fear or is it the desire for change and freedom?” he said, predicting that there would be a significant turnout on the streets. “There is an element of fear, but everyone is saying, ‘We want to go out because we don’t want to lose this.’ ”

There were also more reports of resignations among low-level Baath Party members from several Damascus suburbs, as well as fresh indications that at least some soldiers in Daraa may have defied orders to open fire on protesters. Al-Jazeera aired an amateur video showing what appeared to be uniformed soldiers injured by gunshots who were being tended by civilians, although the contents or origin of the video could not be verified.

Syria experts said they had seen no indications of serious splits within the regime or the military, whose key leaders are drawn either from Assad’s family or his minority Alawite sect.

In the EU meeting in Paris, diplomats were expected to begin debate Friday on possible sanctions against prominent members of Assad’s regime as well as members of his family, said a Western diplomat briefed on the agenda. The sanctions could include a freeze of bank accounts and a ban on travel, the official said.

Sly reported from Beirut.