The remains, which were discovered on Monday, were sent for tests on Tuesday in an effort to identify the bodies, a Karbala police spokesman said.
The men who claim to have discovered the graves were working on underground sewer lines, CBS' Jennifer Donelan reports.
Police will not say how many bodies were found.
Ayad Mashaalah, a local restaurant owner, said he saw bags filled with decomposed bones and corpses. Mashaalah said human rights workers came and took the bags, which are thought to date back to 1991.
BBC estimates that as many as 30,000 people could have been killed and buried in mass graves during the particular Shiite uprising thought to be the source of Monday's finding.
The apparent discovery came amidst ain Iraq, with at least two dozen people — including two U.S. soldiers — killed in shootings and bombings Monday and 18 on Sunday.
In related developments:
Human rights organizations estimate that more than 300,000 people, mainly Kurds and Shiite Muslims, were killed and buried in mass graves during Saddam Hussein's 23-year rule, which ended when U.S.-led forces toppled his regime in April 2003.
Meanwhile, Tuesday, Officials said insurgents were trying to deepen the political turmoil surrounding the contested vote. Preliminary figures have given a big lead to the religious Shiite bloc that controls the current interim government.
The new violence came as three opposition groups threatened a wave of protests and civil disobedience if fraud charges are not properly investigated. The warning came from the secular Iraqi National List, headed by former Shiite Prime Minister Ayad Allawi, and two Sunni Arab groups.
More than 10,000 people, some carrying photos of Allawi, demonstrated Tuesday in favor of a government that would give more power to Sunni Arabs and secular Shiites. Marches chanted "No Sunnis, no Shiites, yes for national unity."
"We're protesting to reject the elections fraud. We want to ask the government and the elections commission: 'Where did our votes go? Who stole them?"' said Abdul Hamid Abdul Razzaq, a 45-year-old barber who attended the massive protest.
A similar protest in Baqouba ended with arrests. Police rounded up several people — most of them high school students — and took them into custody, Donelan reports.
Iraq's Electoral Commission said Monday that final results for the 275-seat parliament could be released in about a week.
Sunni Arab and secular Shiite factions are demanding that an international body review more than 1,500 complaints, warning they may boycott the new legislature. They also want new elections in some provinces, including Baghdad. The United Nations has rejected an outside review.
"We will resort to peaceful options, including protests, civil disobedience and a boycott of the political process until our demands are met," said Hassan Zaidan al-Lahaibi of the Sunni-dominated Iraqi Front for National Dialogue. He spoke in neighboring Jordan, where representatives of the groups have met in recent days.
The election commission considers 35 of the complaints serious enough to change some local results. But Farid Ayar, a commission official, said there was no reason to cancel the entire election.
He also said preliminary results from early votes by soldiers, hospital patients, prisoners and overseas Iraqis showed a coalition of Kurdish parties and the main Shiite religious bloc each taking about a third. Those nearly 500,000 votes were not expected to alter overall results significantly.
Preliminary results previously released gave the United Iraqi Alliance, the religious Shiite coalition dominating the current government, a big lead — but one unlikely to allow it to govern without forming a coalition with other groups.
Alliance leader Abdul Aziz al-Hakim traveled to the northern Kurdish city of Irbil on Tuesday to discuss the formation of a governing coalition with Jalal Talabani, Iraq's Kurdish president, and Massoud Barzani, president of the Kurdish region.
Al-Hakim's secretary Haitham al-Husseini said there would also be negotiations with Sunni Arabs. Al-Husseini said the Alliance has proposed distributing the top six Cabinet positions, the three-member presidency council and top three parliament slots among the political blocs.