Police linked arms and formed a corridor allowing the justices to enter.
The hearing began approximately one hour late. The court's chief justice, Ivan Dombrovskyy, said that 16 of the 18 judges were present.
Yushchenko's allies say the court is too corrupt to render an objective ruling.
"We came because we are afraid that the Constitutional Court is already corrupt, and it won't let people hold elections and kick out this gang [the government led by Prime Minister Viktor Yanukovych]," a pro-Yushchenko demonstrator told RFE/RL. "We will stay until the presidential decree dissolving [parliament] is carried out and elections held."
Yushchenko claimed in a letter to the court that one judge, Syuzanna Stanik, received property worth $12 million in an attempt to influence her decision. Stanik denies the allegation, and Ukraine's Prosecutor-General's Office calls the charge baseless.
Approximately 2,000 Yanukovych supporters were also present at the court building.
On April 2, Yushchenko dissolved parliament and called new elections for May 27, sparking the current showdown.
(with material from news agencies)
RFE/RL's Ukrainian Service asked people on the streets of Kyiv on April 11 whether the Constitutional Court will be able to determine the constitutionality of the president's decree dissolving parliament.
Oksana, a student from Lutsk:
"Their decision will at any rate be beneficial to one of the political forces."
Oleksandr, a high-school student:
"[The court] will be able to do it, but only if the judges agree upon it."
Alla Asilyevna, a pensioner:
"How can the Constitutional Court solve the problem if there is pressure on it from all sides?"
Ivan Yukhimovich, a pensioner:
"If [Prime Minister Viktor] Yanukovych and [President Viktor]Yushchenko find an agreement, everything will be resolved."
Yuliya, a worker:
"I doubt very much that the judges will agree on anything."