BEIRUT, Lebanon — Syrian security forces killed at least 40 people on Friday during antigovernment demonstrations across the country, according to human rights activists, as the government of President Bashar al-Assad intensified a brutal military crackdown that has failed over eight months to extinguish a popular uprising.

Most of the deaths occurred in central Syria, the most restive region in the country, with 21 people killed in Homs and 14 in Hama. Both cities are at the front line of the uprising against the leadership of Mr. Assad and have witnessed mass destruction, arrests and killings since demonstrations broke out. Over all, the United Nations estimates that 3,000 people have been killed since demonstrations began.

The large number of people killed, the most on any Friday since May 6, when 36 demonstrators were shot dead, demonstrated the government’s rejection of international pressure to end the violence, and a determination to rely exclusively on force to silence the sustained challenge to four decades of Assad family rule. Friday — the day of prayer and rest for Muslims — has become the day of protest across Syria, and the Arab world, since the outbreak of popular calls for change.

“They are killing intentionally; they are killing to send a message that they are still in control,” said Omar Idlibi, an activist with the Local Coordination Committees, who lives in Lebanon. “They are committing political suicide. The killings won’t solve the crisis but could lead to international intervention.”