England and defending champions Wales were comfortably victorious but the closest game of the day involved Scotland, who held off a second-half fightback to edge out Japan 19-17.
The Scots were 19-0 to the good a minute into the second half after tries from Chris Dean, Andrew Turnbull and Michael Fedo, with two conversions from Colin Gregor.
Japan hit back through Lote Tuqiri, Shota Emi and Kosuke Hashino, but Katsuyuki Sakai was only able to convert the first try as his side came up short.
Wales opened their defence with a 33-5 win over Uruguay, Chris Knight with two tries. Adam Warren, James Davies and Alex Webber also crossed, with two conversions apiece from Gareth Davies and Rhys Jones.
John Brake scored twice as England beat Portugal 21-7, with Tom Mitchell grabbing the other try. Miguel Lucas' try had levelled the scores at 7-7.
The three teams all lie second in their groups, with Scotland trailing South Africa in Pool B after the latter beat hosts Russia 31-0.
Fiji won 45-0 against Tonga in Wales' Pool E while England's Pool F rivals Argentina beat Hong Kong 47-7.
There were also wins for Australia and France in Pool A, New Zealand and the United States in Pool D and Samoa and Kenya in Pool C.