NEW DELHI—The Indian Supreme Court this week barred lawmakers convicted of serious crimes from serving in national and state legislatures, even if the conviction is being appealed.

Another judgment, also expected to have far-reaching impact, bars politicians who are in prison from contesting elections.

Indians have long decried the fact that legislatures here are packed with politicians charged with crimes. But politicians have long resisted the changes ordered by the court, arguing that the party in power might bring false cases to harass and suppress opposition leaders."This is a first step to break the nexus between criminals and politicians," said Lily Thomas, a lawyer who filed the petition in the Supreme Court in 2005. "The court is cleaning up the politics of this country, which has become highly criminalized."

A spokeswoman for the ruling Congress Party, Renuka Chowdhury, said it would hold consultations with other political parties before charting a response to the judgments, which take effect immediately. "A lot of people are concerned this decision could be misused politically," she said.

The government could ask for the case to be heard by a larger bench of the Supreme Court, which could delay implementation for several years.

Naresh Agarwal of the Samajwadi Party said legislators could also amend the law in Parliament, reversing the court's verdict.

Nearly a third of members of the lower house of Parliament, or Lok Sabha, and 31% of state legislators, have criminal cases pending against them, including kidnapping, robbery, murder and rape, according to the Association for Democratic Reforms, a New Delhi-based advocacy group for transparency in governance. It isn't clear how many have been convicted.

Until the Supreme Court's rulings this week, lawmakers convicted of a crime could delay their disqualification from holding public office by appealing to a higher court—a process that often takes years if not decades as cases crawl through India's overburdened judicial system.

Politicians have also frequently run in elections from their jail cells.

India's political parties follow an opaque process of selecting candidates, rather than using primaries. The Association for Democratic Reforms says its research shows that most Indian political parties, including the ruling Congress Party and the main opposition Bharatiya Janata Party, have frequently picked candidates with criminal records.

The Supreme Court has been a major force behind attempts to reform the political and judicial systems, trying to make them more transparent.

In 2003, the court made it mandatory for candidates running for public office to disclose their criminal histories. Last month, the Central Information Commission ruled that political parties must also comply with the country's Right to Information Act, which allows citizens to seek information from most government bodies.

Activists say the Supreme Court's two judgments this week are the next step in the difficult task of cleaning up India's political system.

"There is a groundswell of support for such steps," said Jagdeep Chhokar, a founder of the Association for Democratic Reforms.

But Mr. Chhokar said the police and judicial systems are still vulnerable to influence by corrupt politicians trying to avoid being arrested or convicted of crimes.

"Politicians will find ways and means of subverting the system," said Mr. Chhokar.

Mr. Agarwal is a general secretary of the Samajwadi Party, which governs Uttar Pradesh, a state where nearly half of state lawmakers have criminal charges pending against them. He warned in an interview of the potential for abuse.

"Two months before elections, the ruling party will bring some case in which we can't get bail for six months," he said. "The whole democratic system will break down."

Mr. Agarwal also said that the move denied voters their choice of candidate.

"In many constituencies, criminals are very popular and get elected again and again," he added.

His party's chief, Mulayam Singh Yadav, and the chief's son, who is the chief minister of Uttar Pradesh, are among top politicians with criminal cases pending against them. The two are being investigated by India's Central Bureau of Investigation over allegations that they own assets in excess of what they have declared.