Human rights groups in Syria say at least 37 civilians were killed during protests to demand a no-fly-zone be imposed over the country, similar to the one over Libya.
The deaths took place mostly in Homs and Hama. At least 170 protests took place on Friday, the usual day of protest.
More than 3000 people have died in the unrest since protests began in March.
NATO planes played a vital role in the overthrow of former Libyan leader Muammar Gaddafi.