Supporters of an anti-tunnel referendum submitted 28,928 signatures to the Seattle City Clerk's office Tuesday morning, trying to ensure the measure advances to a public vote.

The referendum needs 16,503 valid signatures to qualify for the Seattle ballot. City Clerk Monica Martinez Simmons accepted the referendum petitions at 10 a.m. in her City Hall office.

King County Elections will verify the signatures.

Verification is expected to take at least two weeks. Meanwhile, the ordinance it is designed to put to a vote -- agreements between the city and state to proceed on the deep-bore tunnel -- is scheduled to take effect Wednesday.

That means legal action to block the referendum could come as early as Tuesday.

Backers of the referendum argue that the $2 billion deep-bore tunnel leaves the city on the hook for cost overruns, threatens historic buildings in Pioneer Square and will clog downtown streets with traffic as motorists avoid the tunnel tolls that will be needed to help pay for the project.

Supporters of the tunnel say the state's project is the result of 10 years of study, debate and negotiation and should proceed. The Seattle City Council voted to go forward with the project in 2009 and the state signed contracts with a tunnel consortium in January.