The relatives, frustrated by Poroshenko`s failure to bring officials of the previous government to justice, shouted, "Who is a hero for you, Poroshenko?", "Where are their killers?" and "Down with Poroshenko!"
They also attacked him for failing to keep a promise to confer the title of national hero on the victims, which would bring financial benefits to their families.
It was the first real public display of anger against Poroshenko, who was elected president in May after the pro-Moscow Viktor Yanukovich fled the country.
U.S. Vice President Joe Biden, who is currently in Ukraine, had been due to make a visit to the area off Kiev`s Independence Square but called it off, apparently for security reasons.
Friday marked the first anniversary of the decision by the Yanukovich government to ditch a political and free trade agreement with the European Union in favor of renewed trade ties with Kiev`s old Soviet master, Russia.
The move sparked protests from tens of thousands who see Ukraine`s future in the European mainstream. After Yanukovich fled in February, Russia annexed Crimea and backed separatists in the east in a conflict which has killed more than 4,300.
Before the confrontation with the relatives, Poroshenko laid candles with other members of the government on a hill off Independence Square - locally known as the Maidan - where more than 50 of the 100 or so dead were killed in late February.
He was scarcely able to be heard as he tried to speak to the hostile crowd afterward, but said: "If shouting like this continues, everything we did on the Maidan will have been in vain."
He later returned to pledge that he would sign a decree to officially designate the victims, known as the "Heaven`s Hundred", as national heroes as promised.
Elena, 65, whose husband was one of the Heaven`s Hundred, burst into tears.
"I don`t know why it`s taken so long, but now at last they`ve been recognized ... They died so Ukraine could live."
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