In a nationally televised address this evening, Yushchenko said the issue of Piskun serving simultaneously as prosecutor-general and as a member of parliament had caused the current crisis.
"It has been 28 days since Svyatoslav Piskun was appointed prosecutor-general on April 24," Yushchenko said. "The issue [of holding both posts] has not been resolved. Political activities continue at the prosecutor's office. Why isn't the presidential decree on early elections carried out? Because the cabinet is not carrying it out. Why isn't the cabinet carrying out the presidential decree? Because the Prosecutor-General's Office is not working."
Piskun, an ally of Prime Minister Viktor Yanukovych, called his sacking illegal and pledged to defy it.
Ukrainian Interior Minister Vasyl Tsushko, also a Yanukovych ally, called Piskun's dismissal an "attempted coup" and sent police officers to surround and protect his office. Piskun remains there with a number of supporters.
Yushchenko called Tsushko's actions "criminal."
"Who is responsible for establishing legal order in the country?" Yushchenko said. "The prosecutor-general -- a prosecutor-general who is not involved in politics, who does not belong to any faction but ensures the rule of law in an apolitical fashion."
For his part, Piskun told reporters today that he had resigned from his parliamentary post as required.
"I wrote a request asking to resign as a member of parliament and sent it to the Verkhovna Rada within the term prescribed by the law," Piskun explained. "You understand that this decree [on firing him] is absolutely illegitimate and even absurd, because on the 14th of -- well, everybody knows that I wrote this request. All staff members know that."
Yushchenko and Yanukovych have been locked in a power struggle since the president dissolved parliament in April and called for early elections.
In his statement today, Yushchenko assured the nation that the security services and armed forces will not be drawn into the dispute.
"There is one way out of the current situation," Yushchenko said. "It is a political agreement. People with arms will not help resolve this conflict. On the contrary, their involvement in this conflict would delay a political resolution. The manipulation of the work of the Prosecutor-General's Office is delaying a resolution of this issue."
Yushchenko has named Crimea's prosecutor-general, Viktor Shemchuk, as the country's acting prosecutor-general.
Yanukovych cut short a trip to the Crimea and returned to Kyiv to convene an emergency cabinet meeting.