Damascus: Tens of thousands of Syrians are gathering in a central Damascus square for a demonstration in support of President Bashar Assad.

The Syrian president is facing the biggest challenge to his 11-year rule as anti-government protests that began in a southern city exploded across several Syrian cities Friday.

Security forces launched a swift crackdown, opening fire in at least six locations around the country - including the capital, Damascus, and the country's main port of Latakia.

The Syrians in Tuesday's "loyalty to the nation" march carried flags and pictures of Assad.