Protesters gathered in the stands after chanting and waving signs at participants as they arrived for the rally at Safeco Field.
About 20,000 to 25,000 people attended the "Mayday for Marriage" worship service and rally, organized by conservative Christian churches around the state in support of traditional marriage.
"This is a place where we're taking a stand," James Dobson, founder of the evangelical Christian group Focus on the Family, told the crowd. "If this (same-sex marriage) happens, the culture war is over and everything associated with it is lost."
As participants arrived in busloads, they had to walk between sign-carrying protesters lining the sidewalk outside the stadium in this liberal city.
Police estimated 1,500 gay rights protesters showed up outside the stadium, waving signs and chanting, "Bigots go home!" as they marched back and forth out front. A 60-piece marching band performed and passing cars honked in support.
The gay-rights supporters exchanged views with rally participants as police officers worked to control the crowd.
The event was slated as a rally and worship service including music and a choir.
"We believe marriage was designed for a man and a woman," said Bill Hanford, 46, of Redmond. "We think the biblical concept of marriage is important and people should take a stand on that side of the argument as well as the one that's in the media right now."
Among the protesters was Jeffrey Steinberg, 52, carrying a sign that said, "Religious bigots love lies, not God! Bear your false witness elsewhere."
"Unfortunately religion has often been a tool for hatred," he said.
"I read the Bible and pray a lot. Christ accepted prostitutes and tax collectors who were at that time pretty low on the totem pole," Steinberg said. "He never denounced homosexuality in any of the gospels."
Morningstar McKay, 31, came from Everett with her husband and daughter to join the pro-gay rights protests.
"It just makes me really sad because it's really come down to a separation between church and state," she said. "If you fall in love with somebody you should be able to marry them no matter the circumstances."
The Rev. Joseph Fuiten, pastor of the 5,000-member Cedar Park Assembly of God church in suburban Bothell, said local pastors wanted to publicize the importance of traditional marriage.
Fuiten said Saturday's rally was planned over six weeks and churches in the state paid the Seattle Mariners $120,000 to rent the baseball stadium.
The ballpark and Safeco Insurance, whose name is on the stadium, both received calls from people protesting use of Safeco Field for the anti-gay marriage rally, The Seattle Times reported Saturday.
Some threatened to cancel insurance policies while others said they would give up their Mariners tickets, the newspaper said.
The Mariners, who operate the publicly owned ballpark, have taken no position on the gay marriage issue, said Rebecca Hale, public information director for the team.
"This is a facility that was constructed primarily with public funds," Hale said. "We, as operators of the facility, have an obligation to make it available to members of the community for public gatherings. No matter what we think of their political beliefs."
Paul Hollie, a spokesman for Safeco Insurance, said the company does not operate the ballpark. But he said the company has received several comments from employees, clients and others opposed to the rally, calling on the company to intervene to stop it.
"We're not responsible for any of the events," he said.