A flurry of attacks came once the two leaders had been swept up, but none stuck as plenty of teams shared in the pace-setting on the front of the bunch.
It was Orica-Scott, with Mezgec and Ewan, who were in pole position on the technical finale, however, and they reaped the rewards.
Boasson Hagen opened up his sprint first, but Ewan came out of his team-mate’s wheel to snatch victory on the cobbled finale by the smallest of fractions.
OVO Energy Tour of Britain 2017: stage one – result
1) Caleb Ewan (AUS) – Orica-Scott – 4.34.17hrs
2) Edvald Boasson Hagen (NOR) – Dimension Data – ST
3) Elia Viviani (ITA) – Team Sky
4) Alexander Kristoff (NOR) – Katusha-Alpecin
5) Fernando Gaviria (COL) – QuickStep Floors
6) Chris Lawless (GBR) – Great Britain
7) Brenton Jones (NZL) – JLT-Condor
8) Nikolas Maes (BEL) – Lotto-Soudal
9) Zdenek Stybar (CZE) – QuickStep Floors
10) Floris Gerts (NED) – BMC Racing
General classification
1) Caleb Ewan (AUS) – Orica-Scott – 4.34.07hrs
2) Edvald Boasson Hagen (NOR) – Dimension Data +3”
3) Karol Domagalski (POL) – ONE Pro Cycling +4”
4) Elia Viviani (ITA) – Team Sky +6”
5) Alexander Kristoff (NOR) – Katusha-Alpecin +10”
6) Fernando Gaviria (COL) – QuickStep Floors
7) Chris Lawless (GBR) – Great Britain
8) Brenton Jones (NZL) – JLT-Condor
9) Nikolas Maes (BEL) – Lotto-Soudal
10) Zdenek Stybar (CZE) – QuickStep Floors