Yemen's elite Republican Guard forces fired rifles, tear gas, and water cannons on over 100,000 marchers on a road leading to the capital of Sanaa, killing at least 9 and wounding at least 90.
Participants in the so-called "March of Life" followed a 270-kilometre route from the southern city of Taez, a major opposition stronghold. The march reportedly began with some 15,000 participants but grew over four days as it made its way to the capital.
They object to a transfer of power deal agreed to by outgoing president Ali Abdullah Saleh, in which he agreed to step down in exchange for immunity from prosecution.
The agreement, brokered by the Gulf Cooperation Council (GCC), was designed to ease Saleh out of power and avert civil war in a country that has seen a growing Al Qaeda insurgency and sits next to key oil shipping lanes.
Under the deal Saleh officially remains Yemen's "honorary president" until February 21, but the real power rests in the hands of his vice president Abd-Rabbu Mansour Hadi.
Hadi formed a unity government early in December divided evenly between opposition figures and members of Saleh's cabinet. They are expected to run the country until elections are held in early 2012.
But protesters are frustrated with Saleh's slow departure and say he continues to exercise influence behind the scenes.
Witnesses said troops loyal to Saleh spread out across the entrances of streets leading to his compound to block any attempt by protesters to approach it.
The shooting reportedly began as activists entered the Sabaeen district of the capital chanting "no to immunity" at the climax of their four-day march.
The forces responsible for the shooting – which lasted for several hours – are commanded by Salen's son, Ahmed
According to Yemeni officials, Hadi issued orders for the troops to stand down amid the fighting, but they refused until international mediators contacted Ahmed and pressured him to obey.
Saleh is expected to depart for the United States to receive further medical treatment for wounds he sustained in a June assassination attempt.
He said he plans to return to Yemen and remain politically active as an "opposition figure."