Defence ministry spokesman Prasad Samarasinghe said the army had begun enlisting 1,000 soldiers for the special Colombo security unit from Tuesday and 200 had already signed up.
"These men will be deployed only for security duties in Colombo," Samarasinghe said.
"We are also calling for those who retired from the army after 12 years of service to rejoin this special unit."
The recruitment is part of new measures amid fears that the Liberation Tigers of Tamil Eelam (LTTE) could carry out more suicide attacks following the assassination of the army's number three officer last month.
"To protect Colombo after the suicide bombings, a new army volunteer unit with 1,000 soldiers is to be recruited," Samarasinghe said.
Sri Lanka is currently under a state of emergency imposed after the Tigers were blamed for the August 2005 assassination of foreign minister Lakshman Kadirgamar.
The emergency gives police and troops sweeping powers to carry out searches and detain suspects for