London walks: Seeking out the saints in Wren's city: Michael Leapman goes on a tour to see a score of churches designed by the master builder

Billingsgate fish market has long moved from the City to the Isle of Dogs, but the annual 'Harvest of the Sea' service is still held at the Church of St Mary-at-Hill in Lovat Lane, close to the old market building. This year it is on 9 October at 11am. The church is decorated with platters of fresh fish arranged in wondrous patterns - well worth a look even if you are not attending the service.

It was the first church Christopher Wren designed after the 1666 Great Fire and makes a good focus for a walk that takes in 20 of his churches plus St Paul's Cathedral and the Monument.

The weekend is easily the best time for strolling around the City but to look inside the churches choose a weekday, when more are open. Yet even the exteriors and the spires will impress you with Wren's extraordinary range and vitality (given that he was made to churn out dozens of original designs in the space of a few years).

From the station, make for Wren's Monument (1), standing close to the spot in Pudding Lane where the fire broke out. The adjacent temporary murals, shielding a building site, tell the story of the fire graphically. The Monument was on the direct line of approach to London Bridge until 1831, when a new bridge was built a few yards west.

The arch outside the church of St Magnus the Martyr (2), down Fish Street Hill, used to be part of the bridge approach. From St Magnus, turn east along Lower Thames Street, then left opposite the splendid former fish market into Lovat Lane, where St Mary-at-Hill church (3) is on your right. After examining the fish displays continue uphill, turn right on Eastcheap and then left down the street named St Mary-at-Hill. Note the fine clock at the back of the church.

Turn left into St Dunstan's Lane and on your left enter the ruin of St Dunstan-in-the-East (4), never rebuilt after Second World War bombing but made into a pleasant garden. Go through and turn left up Dunstan Hill, left on Great Tower Street and right into Rood Lane for the Church of St Margaret Patten (5), named after a kind of overshoe.

At Fenchurch Street turn left and cross Gracechurch Street into Lombard Street, where you will spot St Edmund the King (6) by its clock. Turn right immediately before the church into picturesque George Yard. Just before the Jamaica Wine House - on the site of London's first coffee house - turn right into an alley leading to the garden in front of St Michael Cornhill (7). Through the garden, follow the passage into Bell Inn Yard and turn left up Gracechurch Street. Just opposite the splendid Leadenhall Market turn left up narrow St Peter's Alley, leading to St Peter upon Cornhill (8) with its red brick tower. Follow the alley round the church and turn left at Cornhill, then left into Ball Court.

By the George and Vulture turn right and follow the maze of passages through Change Alley, emerging on Lombard Street. Turn right and cross the Street in front of St Mary Woolnoth, by Wren's pupil Hawksmoor. Go down St Stephen's Lane and immediately turn right at Mansion House Place, then left to reach the side of St Stephen Walbrook (9), the Lord Mayor's parish church and one of Wren's finest.

Walk ahead up Bucklersbury and go left on Queen Victoria Street, passing the foundations of the excavated Roman temple. On your right you catch a glimpse of the dome of St Paul's. Cross Queen Street to St Mary Aldermary (10), then go right up the narrow Bow Lane to St Mary-le-Bow (Il), with its famous bells.

Turn left on Cheapside towards St Paul's (12) and head up Newgate Street to see the ruin of Christ Church (13) on your right. Ahead again, you pass Old Bailey and cross Holborn Viaduct to reach St Andrew Holborn (14), with its pretty figures of blue-coated children outside.

Walk south down Shoe Lane making for the wedding-cake spire of St Bride's (15), across Fleet Street. Leaving the church by Bride Court, passing the Bell Tavern, turn right then left through a passage to New Bridge Street. Cross the street and go up steps in Pilgrim Street to emerge on Ludgate Hill opposite St Martin Ludgate (16).

Turn right, then immediately right into Ludgate Square, heading downhill towards St Andrew's Hill and St Andrew-by-the-Wardrobe (17) - the wardrobe being the old royal supply depot sited here in the Middle Ages. Head east up Queen Victoria Street making for the red brick St Benet's (18) on your right, now used by the Church of Wales.

From here those who want to

maximise the church count can head up Queen Victoria Street to St Nicholas Cole (18a). The nicer walk

follows the signs to the riverside path and head east by the Thames, past a fine view up to St Paul's through a gap in the buildings. On the opposite bank, next to the reconstruction of Shakespeare's Globe, is the little white house from which Wren is said to have watched his cathedral being built.

When the path emerges on Upper Thames Street, turn right and cross the road to St James Garlickhythe (19). Walk behind the church along Skinner's Lane then ahead to College Street, passing St Michael Paternoster Royal (20) on your left. Turn left up Dowgate and right in front of Cannon Street Station. Cross Cannon Street and turn left up Abchurch Lane to St Mary Abchurch (21).

Keep ahead to King William Street and turn right, passing St Clement Eastcheap (22) on your left. Now, if you have the energy, climb the Monument's 311 steps for a bird's eye view of all the churches.

FACT BOX

Length: About three miles

Time: More than two hours, plus time to look at the churches

Underground: Monument (District and Circle Lines)

Buses: Numerous routes to Bank

Parking: Inadvisable on weekdays. Meters and single yellow lines at weekends.

(Photograph omitted)