The sheriff of Middlesex County, Mass., announced Monday he will run for the Fifth District congressional seat being vacated by Rep. Ed Markey, the Democrat elected to the U.S. Senate in a special election last week.

Peter Koutoujian, a Democrat, announced his candidacy online, declaring that if he is elected he will be “a strong progressive leader” who will fight for abortion-rights legislation, a ban on assault weapons and measures that address climate change and immigration reform.

“We need our next representative in Congress to be someone who will do the right thing when it comes to growing the economy, creating jobs and improving education and the environment,” Koutoujian said.

A descendent of Turkish immigrants, Koutoujian was appointed sheriff in 2011 by Democratic Gov. Deval Patrick. Koutoujian is also a former state representative who helped pass anti-stalking legislation in the Bay State as well as a law that raised the minimum age for operating an all-terrain vehicle from 10 to 14 years old in response to a series of deadly ATV crashes involving children.

The Ffith District encompasses several cities adjoining Boston as well as Revere, Southborough, Sudbury and Winchester.