Attackers hurled a grenade at British tourists in Mombasa, Kenya, but it failed to explode, according to the AFP news agency.
Robert Kitur, police chief for the port city, said: "The grenade was detonated safely by experts.
"We are investigating the incident and looking for the man who threw the grenade at the tourists and fled."
The tourists, who were in a vehicle in the Likoni district of Mombasa, were travelling from the popular Indian Ocean resort of Diani on the way to a wildlife safari at the Masai Mara national park.
The British High Commission in Kenya said it had "heard the reports and was investigating", but gave no further details.
Since Kenya invaded southern Somalia in October 2011 to try to oust al Qaeda-linked al Shabaab insurgents, it has seen a wave of grenade attacks.
The explosives have been thrown into restaurants in Mombasa and crowded areas in the capital Nairobi, as well as a string of attacks in the remote northeast region bordering Somalia.
Al Shabaab claimed responsibility for the assault on Nairobi's Westgate mall in September, in which at least 67 people died during a four-day siege.
In September 2011 gunmen killed British man David Tebbutt and kidnapped his wife Judith. She was held for six months before being released.