More than 2,000 people, including police officers who traveled from as far away as Massachusetts, attended the service for Smith, 42, at the Neighborhood Church of Castro Valley.
Smith, a San Ramon resident, worked for BART for more than two decades and was known as "Tommy."
He was accidentally shot and killed by a fellow BART officer, Det. Michael Maes, when they were among a group of officers conducting a probation search at a robbery suspect's apartment at 6450 Dougherty Road in Dublin at about 2 p.m. on Jan. 21.
Newark police Officer Patrick Smith, one of Smith's older brothers, said Smith loved his wife Kellie, who is also a BART police officer, "more than life itself" and said the birth of their daughter Summer "was his greatest achievement."
Patrick Smith remembered his brother as "a quick-witted personality" and said "his life ended far too early and his early departure has cheated all of us."
California Attorney General Kamala Harris told Summer, "Your dad is a hero and that is how the state of California thinks of him and will always remember him."
BART police Chief Kenton Rainey said Smith joined the transit agency's police force as a cadet at the age of 18 and spent his entire career there. Smith is the first BART officer to be killed in the line of duty in the agency's 42-year history.
Officers from throughout the Bay Area and California as well as from other states attended Smith's service.
Lt. Michael Rae of the Massachusetts Bay Transit Authority said he and four colleagues from the Boston-based agency "came out of respect for our brother in uniform. Sgt. Smith and his colleagues are in our hearts and we grieve for them," Rae said.
Contributions can be made to Smith's family by donating to the Tommy Smith Memorial fund at any Wells Fargo bank branch. Donors should reference account #5148561086 under the name of Kellie Smith.
Donations can also be mailed to the Tommy Smith Memorial Fund c/o Wells Fargo, 11020 Bollinger Canyon Road, Suite 1, San Ramon, CA, 94582.