Dumitrescu died at home two weeks after he was released from hospital where he was being treated for liver disease, national news agency Agerpres reported.
After communism ended in 1989 with the overthrow and execution of dictator Nicolae Ceausescu, Dumitrescu was elected to Parliament where he fought as a senator for the exclusion of former communists from public life. He helped draft laws that led to the publication of the millions of files of the feared Securitate secret police files.
He was widely praised for his role in helping Romania move on from its difficult past, particularly as ex-communists continued to rule the country until 2004.
"We have lost one of the most powerful voices against communism," said President Traian Basescu.
"The departure of ... the one who remained upright and dignified during thousands of days of prison and interrogation faced with the challenges and methods of interrogating, humiliation and compromise," is a sad day for all Romanians.
"If Romanians have access to their Securitate files today ... this is largely due to Dumitrescu," said Prime Minister Calin Tariceanu Popescu who knew him well.
Dumitrescu was sentenced to 27 years prison in 1949 for being an enemy of the state. From 1949 to 1964, he was incarcerated in communist jails or kept under house arrest.
After he was released he was not allowed to practice his profession as a lawyer, and was forced to work in construction, a typical career path for former dissidents.
He retired from the Senate in 2000, and in May published his memoirs, "Witness and Document," detailing his time in a communist gulag. He was also chairman of the Association of Former Political Prisoners.
During the communist era, hundreds of thousands of Romanians were imprisoned or sent to labor camps for political reasons.
Romania is slowly addressing its communist past. Basescu officially condemned communism in a 2006 report and denounced it as "criminal and illegitimate" and guilty of crimes against humanity. This year two political detainees were awarded substantial compensation for their time in prisons.
There was no immediate word about survivors, or funeral arrangements for Dumitrescu.