Gothabaya Rajapakse (l) is hugged by his brother after the blast
Gothabaya Rajapakse, who is also the defence secretary, was "safe", a military spokesman said.
The government has blamed Tamil Tiger rebels for the blast, which killed at least one person and hurt 14 others.
The spokesman said a suicide bomber triggered the explosion near a convoy of vehicles carrying Mr Rajapakse.
Brig Prasad Samarasinghe said Mr Rajapakse was in one of the cars.
"He is safe, no harm has come to him," he said.
The BBC's Dumeetha Luthra in Colombo says Gothabaya Rajapakse has been a vocal critic of the rebels, and is known as a hardliner who is against holding talks with them.
An eyewitness told Reuters news agency that he saw a wreckage of a taxi and a pick-up truck at the blast site.
'No option'
Senior police official Jayantha Wickremeratne said the suspected bomber was apparently riding a scooter rickshaw and targeted the convoy of vehicles.
It was not immediately known who was responsible for the bombing.
A convoy of cars was targeted by the suspected bomber
President Rajapakse said in statement that the Tamil Tigers were known to have carried out such cowardly and brutal attacks before.
The president "remains unshaken in his resolve to achieve peace in Sri Lanka and is undeterred in his efforts to combat all forms of terrorism and violence," a statement from his office said.
The explosion comes days after the Tamil Tiger rebels said they had "no option" but to push for an independent state.
Rebel leader Prabhakaran had said a truce with the military was "defunct" and accused the government of unleashing war on Tamils.
Correspondents say that while he did not mention re-starting the war, the threat was there in almost every sentence.
Violence has soared in Sri Lanka since late last year, with both sides accused of breaking the 2002 ceasefire.
About 65,000 people were killed in fighting before the 2002 truce was agreed.
The rebels want a homeland for minority Tamils in the north and east. They say Tamils have been discriminated against by the island's Sinhalese majority.
The Sri Lankan Defence Secretary, Gothabaya Rajapakse narrowly escaped a suicide attack from suspected Liberation Tigers of Tamil Eelam (LTTE) rebels. The attack took place in downtown Colombo, the Sri Lankan Capital at 10:30 a.m., as a convoy carrying the Defence Secretary and other officials was attacked by a suspected terrorist in a three wheeler. 

Early reports indicate that Gothabaya, brother of Sri Lankan President Mahinda Rajapakse and vocal critic of the LTTE, escaped unscathed, while at least 2 people have died and 14 others were injured, including 5 civilians. 

While the LTTE has yet to claim responsibility for the latest strike, the militant separatist group, proscribed as a terrorist organisation in countries such as the United States, England, India, Australia and the E.U., has yet to do so for any of the suicide attacks attributed to them throughout their history. In 2006 alone the group has been accused of carrying out suicide attacks on unarmed Sri Lankan servicemen in Habarana and on a naval base in the tourist city of Galle, as well as failed attempts on the lives of Lieutenant General Sarath Fonseka and Pakistani Ambassador to Sri Lanka, Bashir Wali Mohamed. 

The latest suicide attack comes just days following a declaration from LTTE leader, Velupillai Prabhakaran, that the Norway brokered ceasefire agreement with the Sri Lankan government was ‘defunct’.