The former STUC general secretary, who died last week aged 57 after a long illness, was described as a "great oak" of modern politics.
First Minister Alex Salmond and his predecessor Jack McConnell were among the senior political figures who turned out to pay tribute.
Scottish Labour leader Iain Gray, former Scottish Socialist Party MSP Tommy Sheridan, Respect MP George Galloway and former Scottish Labour leader Wendy Alexander all attended the service at a packed Renfrew Town Hall.
Mr Speirs joined the organisation as an assistant secretary in 1979. He became deputy general secretary in 1988 and general secretary ten years later.
The town hall was filled with former colleagues, fellow activists, family members and well wishers.
Mourners, including Mr Sheridan and Ms Alexander, spilled onto the pavement outside the hall in order to pay their respects.
Mr Galloway said that "a great oak has fallen in the political forest" with the death of Mr Speirs, while Mr McConnell said he was "proud to have known and learned from Bill".