By Zach Howard

BOSTON (Reuters) - A former Massachusetts crime lab chemist accused of falsifying evidence tied to as many as 34,000 cases pleaded not guilty to obstruction of justice charges on Wednesday.

Prosecutors allege Annie Dookhan, 35, tampered with drug evidence and faked test results at the Hinton State Laboratory Institute in Boston, where she had worked for nine years analyzing drug samples submitted by law enforcement across the state.

Dookhan is accused, among other things, of altering substances in vials to cover up her practice of visually identifying samples without doing the proper chemical testing.

Dookhan was arrested in September and subsequently charged with tampering with evidence, obstruction of justice, perjury, and falsely claiming to have an advanced degree in chemistry. Prosecutors charge she got the job by falsely claiming she had a master's degree from the University of Massachusetts.

Investigators have said they identified some 10,000 people convicted or accused of crimes based on evidence she had handled at the Hinton lab, which is temporarily closed.

State officials say 252 people had been released from prison pending new trials as a result of the investigation, as of January 4.

Dookhan pleaded not guilty to five obstruction of justice charges during an arraignment on Wednesday at Plymouth County Superior Court's criminal division in Brockton, Massachusetts. A separate arraignment was set for Wednesday afternoon at another courthouse.

Dookhan was indicted by a grand jury on 27 charges in mid-December.

Authorities believe she gathered samples from different cases that appeared to contain the same substance, but tested only a few of them. She allegedly tampered with vials undergoing subsequent testing to make sure they were consistent with her initial findings, they have charged.

The release of prisoners has created a headache for state and local officials. Boston Mayor Thomas Menino in November requested $15 million in state aid to help the city cope with additional police and shelter expenses that resulted from the release.

Dookhan, who told officials according to a police report that she "screwed up big time," has said that no one else at the lab knew what she had done and that she was just trying to get more work done.

This month, a second state crime lab chemist who worked at the Amherst lab in western Massachusetts was charged with tampering with evidence and drug possession after being accused of stealing already-tested illegal drugs for her own use.

(Editing by Paul Thomasch and Tim Dobbyn)