As organizers from the Council on American-Islamic Relations began Thursday's event, a woman grabbed a microphone erected on the Capitol steps and proclaimed the name of Jesus Christ.
"Islam will not dominate the United States and by the grace of God it will not dominate Texas," the woman yelled.
The microphone was wrestled back and the rally continued peacefully. But nearby hecklers yelled things like "assimilate or go home."
Council Executive Director Mustafaa Carroll said the event began in 2003 and never drew opposition before. But this year, online threats prompted the council to contact the FBI.
Added police presence around the event was also conspicuous.
Last week, hundreds of protesters gathered outside a Muslim group's conference in Garland and heckled participants.
The Associated Press contributed to this report.