"There is a general security alert across Colombo," police spokesman Jayantha Wickremaratna said.
Officials have blamed the bombs on Tamil Tiger rebels, saying they were targeting civilians in revenge for military defeats.
"In the midst of repeated defeats and setbacks in the battlefield, LTTE attacks against civilians could intensify in the coming weeks," Wickremaratna said, referring to the Liberation Tigers of Tamil Eelam.
"The LTTE has targeted innocent civilians who have no involvement in the ongoing military operations," he said.
England's cricket test series in Sri Lanka would go ahead despite the attacks, team spokesman Andrew Walpole said.
"There has been no change in the travel advice we have received from them (the British High Commission in Colombo) at this stage, and it has no effect on the tour arrangements," Walpole said.
The High Commission reminds Britons "there is a high threat from terrorism.
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"It is for the team to decide... but we have told them o