The casualties were all farmers who had gathered in the area to receive free seeds from the government as part of a program to encourage them not to plant opium poppy, Daoud Ahmadi, a spokesman for the provincial government, said.
Lashkar Gah is near Marjah in Helmand province, the focus of a major Nato-led offensive against the Taliban.
Thousands of Nato and Afghan troops have launched a massive offensive in southern Afghanistan - the largest the country has seen since the 2001 US-led invasion.
A major offensive in Kandahar, once a Taliban stronghold, would follow the current military operation in neighbouring Helmand province,which appears to have largely pushed back the Taliban and given the government a chance to take control.
Marjah is also thought to be the hub of the Taliban-controlled opium trade - which provides them with most of their funding.
The Afghan defence ministry has said the anti-Taliban push will be led by Afghan security forces as part of plans to hand over military and police responsibility to the Western-backed government.
The US and Nato have about 113,000 troops fighting in Afghanistan, with another 40,000 being deployed in the coming months.