Beneath the bridge, crowds had gathered on the banks of a stream to mark Nowruz, the Zoroastrian new year which falls on the March 21 equinox.
Helmand province has been the scene of some of the heaviest fighting between foreign forces and Taliban in recent weeks.
Last month, about 15,000 US, Nato and Afghan troops staged a major offensive to take control of the town of Marjah , south of Lashkar Gah.
Taliban forces in Marjah scattered in the face of the massive allied assault but are fighting back with ambushes and hidden bombs.
Local residents have said the Taliban remain strong throughout the province despite the offensive in Marjah.
In eastern Khost province, which borders Pakistan, a roadside bomb killed two construction company guards when it hit their car on Sunday.
Three other people were injured in the blast, Amir Badshah Rahmatzai Mangal, the head of the provincial public health department, said.
Yaqoub Khan, the police chief in Khost, was about 40 metres away from the
blast but was not injured.
He said Afghan and US forces provided enough security to prevent a deadlier attack
"but unfortunately the enemy uses every means and planted a bomb just off the road."
On Saturday, two explosions shook the city of Jalalabad in Nangarhar province north of Khost, causing no damage or casualties.
Ghafour Khan, the Nangarhar police spokesman, said the bombs went off in a
thinly populated area and were designed "to create fear among the people'' who were attending concerts and public events marking the new year.
There are some 120,000 foreign troops in Afghanistan and that is set to rise to nearly 150,000 by the end of this year as Washington sends in more troops as part of a new strategy to try and quell the mounting violence.
Taliban attacks are the biggest killers of civilians in the Afghan war, according to the UN, with roadside bombs and suicide attacks indiscriminate in their collateral damage.