Thousands of Guatemalans protested Sunday to demand the president resign over accusations that he ordered a lawyer killed, a scandal threatening the rule of the country's first leftist leader more than 50 years.

Supporters of President Alvaro Colom staged a counter-demonstration. Many were farmers and workers who have benefited from his social programs.

Colom denies Rodrigo Rosenberg's allegations, which were broadcast posthumously after the attorney was shot to death a week ago. He has dismissed calls for his resignation and asked the FBI and a U.N. panel to investigate the killing.

"Get out! Get out!" protesters chanted at the headquarters of the Guatemala City government. Dressed in white, many were from the middle and upper classes that make up the bulk of opposition to Colom.

"This is a civic movement of Guatemalans who are seeking peace. We are tired of violence, ineptitude and corruption in Guatemala," said Jorge Briz, the president of the country's chamber of commerce.

At the plaza in front of his offices, Guatemala City Mayor Alvaro Arzu lowered the Guatemalan flag to half-staff and raised a black flag to mourn Rosenberg.

Protesters said they collected more than 25,000 signatures for a petition to demand that Congress strip Colom of his immunity from prosecution.

The demonstrators canceled a 15-block march to the National Palace to avoid colliding with the thousands of Colom supporters who filled the nearby Constitution Plaza and spilled over into the surrounding streets.

"He's staying! He's staying! The president is staying!" the Colom backers chanted. Many were from the impoverished countryside that has been a stronghold of support for the embattled leader.

"He's with the poor," said Juan Gonzalez, who traveled to the capital from rural Guatemala.

A recording distributed at Rosenberg's funeral showed him blaming his death on the president, first lady Sandra Torres and Cabinet chief Gustavo Alejos, who also proclaim their innocence.

Torres has also been at the center of corruption allegations against the Social Cohesion Cabinet, the backbone of Colom's social programs, including direct payments to rural families for sending their children to school and food rations for urban slum dwellers. She denies the allegations.

In the video, Rosenberg said officials might want to kill him because he represented businessman Khalil Musa, who was slain in March along with his daughter. The lawyer said Musa, who had been named to the board of Guatemala's Rural Development Bank, was killed for refusing to get involved in purported illicit transactions at the bank.

The video was shot in the office of journalist Mario David Garcia, who says he tried to persuade Rosenberg to denounce what he knew on the air but ran out of time.

In response to Colom's request, an FBI agent arrived in Guatemala last week to coordinate with local prosecutors and with the International Commission against Impunity in Guatemala, a U.N. panel set up in 2007 to clean up corruption.

Colom's 2007 election victory gave Guatemala its first leftist leader since Jacobo Arbenz was thrown out of office in 1954 by a CIA-orchestrated coup.