Friday's announcement, which makes the Harvard-trained Johnson Sirleaf the first elected woman president in Africa, was made on the basis of 97% of votes counted after Tuesday's second round of polls in the west African country.
She took 59.4% of the votes with the 97% counted, according to the NEC, which rendered her rival George Weah, a former football star, unable to catch up on the basis of his current 40.6%.
The two were the front-runners who emerged from a first round of elections in October to choose a head of state in a nation where a two-year political transition followed successive and brutal civil wars.
Earlier on Friday thousands of Weah's supporters hit the streets of Monrovia to denounce vote results they claim were fraudulent.
They marched from his party headquarters past the NEC area of the seaside capital, even as Weah made an appeal for calm to hundreds of other partisans still milling around the compound.
Weah accused his opposition of
"I can see in your eyes, I can see in your faces that you are crying but there is no need to cry because we have not lost the election," he told the crowd earlier.
Police officers from the UN peacekeeping mission in Liberia shared patrol duties with Liberian National Police clad in riot gear, keeping the marchers confined to one side of the road that bisects the city.
They waved branches and signs, and chanted "No Weah, no peace".
The protest follows a petition by Weah's Congress for Democratic Change party to the Liberian Supreme Court to stop counting votes from Tuesday's run-off, which gave Johnson Sirleaf a 59.1% lead.
The former FIFA player of the year has alleged ballot tampering, intimidation and harassment among the fraudulent acts conducted during the run-off vote on Tuesday in which Johnson Sirleaf earned 59.1% of more than 90% of votes cast, according to tallies by the NEC.
Weah's supporters denounced
results claiming fraudulence
A host of international observers declared Tuesday's elections to be peaceful, while also noting some irregularities, though most refrained from anointing them with the coveted "free and fair" label until all results were in.
Liberia's electoral process, mandated under a peace agreement signed in August 2003 to end the second of two civil wars, unrolled in a largely peaceful and enthusiastic atmosphere.