Syrian security forces have shot dead two people and launched a campaign of arrests just hours after the United States and its European allies demanded that president Bashar Assad leaves office, activists said today.

The overnight shooting and arrests in different parts of the country came as the opposition called for more demonstrations following noon prayers today.

The London-based Syrian Observatory for Human Rights and The Local Co-ordination Committees, a group that documents anti-regime protests, said one person was killed in a Damascus suburb. Another died early today in the central city of Homs.

Human rights groups and witnesses say Syrian troops have killed more than 1,800 civilians since mid-March.

A UN team has said Assad's crackdown "may amount to crimes against humanity".