The blast in the southern province, which supplies much of the opium used in the world's heroin trade, followed an attack Wednesday that killed two Canadian soldiers and wounded a radio journalist.
Thursday's bomb was triggered by remote control in the town of Gereshk when Helmand police chief Mohammad Hussein passed by in a convoy of several cars, said Hussein, who was unhurt in the attack. Five of the wounded were in critical condition, he said.
On Wednesday, a suicide bomber rammed his car into a convoy of vehicles carrying the governor of eastern Khost province, killing four people and wounding eight others. The governor was not hurt.
A local television station, meanwhile, broadcast a video of a German engineer kidnapped by Taliban insurgents more than a month ago, showing him pleading for help. The man, who identified himself as Rudolf Blechschmidt, was shown lying on a black rug, clutching his chest and coughing.
"I am a prisoner of the Taliban," he said. "We live in the mountains, very high in a very bad condition, please help us.
"The Taliban try to negotiate with the Afghan government but the government not talk with the Taliban and the Taliban tried to get in connection with the embassy to release us. But if the time is over, they want kill us," said Blechschmidt, speaking in broken English.
The video was broadcast on privately owned Tolo TV. The station did not say how it obtained the footage, and there was no indication of when it was shot.
The German Foreign Ministry in Berlin said they are checking the content of the video.
"The Foreign Ministry's crisis task force is carefully analyzing the video," a spokeswoman for the Foreign Ministry said on customary condition of anonymity. "It continues to work intensively to secure the hostage's release."
Abductions have become a key insurgent tactic in recent months in trying to destabilize the country, targeting both Afghan officials and foreigners helping with reconstruction efforts. A group of 23 South Korean aid workers were taken hostage last month. Two of the Koreans were killed, two were released and the rest remain captive.
Blechschmidt is one of two German engineers and five Afghans taken hostage on July 18 in Wardak province in central Afghanistan. The other German was found dead of gunshot wounds on July 21, while one of the Afghans managed to escape.
The captors have demanded in the past that Germany withdraw its troops from the country.
Violence in Afghanistan is currently running at its highest level since U.S.-led forces invaded the country in 2001 to oust the hard-line Islamic Taliban rulers who were accused of harboring al-Qaida leader Osama bin Laden following the Sept. 11 attacks.
The Canadian soldiers were traveling in an armored vehicle that was hit by a roadside bomb Wednesday in Zhari district of Kandahar province, NATO Brig. Gen. Guy Laroche told reporters at Kandahar Airfield.
Wednesday's casualties - who were from Quebec province's Royal 22nd Regiment - bring to 69 the number of Canadian soldiers killed in Afghanistan since 2002. Canada has about 2,300 soldiers in the country, mainly operating in Kandahar province, the former Taliban stronghold.
The Afghan mission is unpopular in the French-speaking province and rising casualties have cost Canadian Prime Minister Stephen Harper's Conservative government support there. Harper has said Canada's combat mission in Afghanistan will not be extended beyond 2009 without a consensus in Parliament.