The president can extend the emergency for a period of one month at a time, but it must be ratified by parliament within 10 days.
Sri Lanka has faced criticism over the use of emergency laws, which were imposed in 1983 to combat the Tamil Tigers.
The emergency allows the arrest and detention of suspects for long periods without trial, as well as allowing police and troops to carry out search operations without a warrant from a magistrate.
|Fonseka aims to contest parliamentary elections despite his detention [Reuters]
Lucien Rajakarunanayake, the media director of the Sri Lankan presidential office, told Al Jazeera that "security concerns" had prompted the government to extend emergency powers".
"There is really no issue here for concern, because the government has done this before," he said.
The government also argues that although the Tamil Tigers have been defeated, rebel remnants are trying to make a comeback, a claim rejected by Sri Lanka's opposition which says the emergency is being used to suppress dissent.
Rajapaksa dissolved parliament last month and called for legislative polls on April 8 after winning the country's presidential election in January.
The head of Sri Lanka's opposition, defeated presidential candidate and former army chief General Sarath Fonseka, is still being detained in military custody accused of plotting to overthrow the government.
Despite his detention he has announced he will nonetheless stand as a candidate in next month's parliamentary elections.
Last week, he signed papers to contest a seat in the capital Colombo, representing the Democratic National Alliance.
However, Sri Lanka's opposition is weaker than it was during January's presidential polls because the coalition that supported Fonseka's candidacy is now split into several blocs.