At Junction 18 (Charing Cross) the motorway reduces temporarily to two lanes in each direction. This bottleneck severely limits the capacity of the M8 through the city centre and contributes to the congestion which has been present since the summer of 1980. Turning southwards, the road proceeds through Charing Cross in cutting, passing beneath Sauchiehall Street in an underpass. The road passes the Mitchell Library and under several other streets, still with only two lanes for through traffic. After passing Junction 19 (Anderston) and a connection the Clydeside Expressway, the road rises as it approaches the iconic Kingston Bridge. The structure, which is ten lanes wide, is one of the busiest urban road crossings in Europe and is used by over 150,000 vehicles every week day. The Charing Cross section of the motorway, completed in early 1972, was the most controversial of all the M8 projects completed. Several thousand homes, in varying states, were demolished for the Comprehensive Development Areas, of which the Inner Ring Road was a key part. The Kingston Bridge has been extensively refurbished since the mid-1990s, with some slip roads reconstructed and the main bridge strengthened. Incomplete slip roads at Junction 20 (Tradeston) are a visible reminder of the intended connection to the unbuilt South Flank of the Inner Ring Road.

After Junction 20, the motorway sweeps westwards, skirting between Scotland Street and Tradeston. The section from here to Junction 26 (Hillington) was constructed in two concurrent stages as the Renfrew Motorway from 1973 to 1976. The Stage 1 contract, which extended to Junction 24 (Helen Street), is most famous for its braided carriageways. Here, 16 lanes of traffic provide connections between the M8, M74 and M77 motorways. It was designed to reduce weaving traffic and improve safety. It remains the second widest stretch of motorway in the UK, beaten only by the M60/M61 Interchange in Manchester. The connection with the M74 was completed in 2011, slightly west of the originally planned link with the South Flank of the Inner Ring Road.

Another key feature of this section is the Scotland Street Viaduct, provided to cross the now closed railway line to General Terminus Quay. At Junction 22 (Plantation), the M77 heads south via two curved ramps. The number of traffic lanes reduces to four lanes with intermittent hard shoulder in each direction. At Junction 23 (Ibrox) traffic from the M74 finally merges with the M8 mainline. Access to the junction is restricted to M74 traffic only for safety and weaving reasons. From Junction 24 the speed limit increases to 70mph. Stage 2 of the Renfrew Motorway extended from Helen Street to Hillington, with a major interchange provided at the A739 Clyde Tunnel Expressway at Junction 25 (Cardonald). The road varies in width from three to four lanes wide with hard shoulders as far as Junction 26. At Junction 25 some slip roads are particularly wide or have large dead areas. This is a visible reminder of the intended connection with the South Link Motorway, a route that would have provided connections to the south of the city, as well as east to south movements to the M77 from the M8. Junction 25a (Braehead) was constructed in 1998 to serve the new shopping centre and industrial development. A two-lane spur leaves the motorway and ends on a signalised junction adjacent to the shopping centre main car park. In recent years, congestion has become a major problem on this part of the route with peak time delays experienced on most weekdays.