Yemen began enforcing a temporary ban on motorbikes in the capital on Sunday to prevent "shoot and scoot" attacks as Al-Qaeda suspects on a bike elsewhere killed an army officer.
The interior ministry said the ban will last until December 15 and that its aim is to "preserve security and stability" as Yemen undergoes a difficult political transition.
An AFP correspondent in Sanaa reported that the ban was being strictly enforced, with no motorcycles seen on the streets on Sunday.
Motorbikes are a cheap form of transport frequently used as taxis in the impoverished Arabian Peninsula country.
But they have also become a tool for hit-and-run shootings that have killed dozens of officials.
Hundreds of demonstrators protested on Saturday in Sanaa against the bike ban near President Abdrabuh Mansur Hadi's residence, where the authorities used water cannon and tear gas to disperse them.
Al-Qaeda in the Arabian Peninsula (AQAP) has been blamed for most of the motorbike attacks on members of the security forces.
As the ban took effect in Sanaa, gunmen on a motorbike killed the deputy commander of the 37th brigade in the southeastern province of Hadramawt.
"Gunmen on a motorbike opened machinegun fire on Colonel Ahmed Hassan al-Marfadi's car killing him and his son and wounding one of their three bodyguards," a military official told AFP.
The official said the attackers, who managed to escape after the assault in the town of Qoton "are believed to be members of Al-Qaeda".
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On Tuesday, two gunmen on a bike killed a Belarussian defence contractor and wounded another as they left a Sanaa hotel.
And on November 22, Abdel Karim Jadban, a parliamentarian representing Zaidi Shiite rebels who fought a decade-long insurgency in Yemen's far north, was shot dead in a similar attack.
A security official in the capital, Colonel Yehya al-Akouaa, told AFP the Sanaa ban aims to "prevent further attacks amid fears of an upsurge in such attacks as the national dialogue nears its end".
Yemen has been going through a difficult political transition since veteran president Ali Abdullah Saleh was ousted in February 2012 after a year of deadly protests against his 33-year rule.
The transition is expected to culminate in a new constitution and pave the way for parliamentary and presidential elections slated for February 2014, but it still faces many hurdles.
There are growing demands for the secession of the formerly independent south, in addition to on-off fighting in the far north between the Shiite rebels and hardline Sunnis.
Yemen is also battling AQAP, which Washington regards as the jihadist network's most dangerous branch.
AQAP often attacks members of the security forces, despite suffering setbacks in a major army offensive last year and repeated US drone strikes targeting its commanders.
In January, the authorities impounded 500 illegal motorbikes during a three-day campaign aimed at getting rid of bike-borne gunmen.
There are estimated to be more than 200,000 motorbikes in the country, and police in Sanaa said at the beginning of the year that most of those machines were unregistered.