Houston: Four former US presidents are among about 1500 mourners gathering at a Houston church for the funeral of former first lady Barbara Bush, the matriarch of one of the country's most prominent political dynasties.

Flags were flown at half-mast for the wife of the nation's 41st president and mother of the nation's 43rd. The first attendees arrived on city buses escorted by Houston police motorcycle units on Saturday morning, and walked into St Martin's Episcopal Church in Houston in a light drizzle of rain.

They included former Rep. Gabby Giffords and her husband, retired astronaut Mark Kelly, and professional golfer Phil Mickelson, along with Karl Rove, and other former White House staff.

About 1,500 people are expected at Saturday's private service, including four former presidents: Bush's husband, George HW Bush, and their son, George W Bush, along with Bill Clinton and Barack Obama.