COVINGTON, La. - BP started pumping heavy mud into the leaking Gulf of Mexico well yesterday and said everything was going as planned in the company's boldest attempt yet to plug the gusher that has spewed millions of gallons of oil over the last five weeks.

BP hoped the mud could overpower the steady stream of oil, but chief executive Tony Hayward said it would be at least 24 hours before officials know whether the attempt has been successful. The company wants to eventually inject cement into the well to permanently seal it.

"I'm sure many of you have been watching the plume," Hayward said from Houston. "All I can say is it is unlikely to give us any real indication of what is going on. Either increases or decreases are not an indicator of either success or failure at this time."

The stakes are high. Fisherman, hotel and restaurant owners, politicians and residents along the coast are fed up with BP's so-far- ineffective attempts to stop the oil leak that sprang after an offshore drilling rig exploded April 20. Eleven workers were killed, and by the most conservative estimate, 7 million gallons of crude have spilled into the Gulf, fouling Louisiana's marshes and wildlife.

The top kill has worked above ground but has never before been tried 5,000 feet beneath the sea. Company officials peg its chance of success at 60 to 70 percent.

President Obama said there are "no guarantees" that it would work. He plans to visit Louisiana tomorrow.