The Scottish cabinet will today discuss plans to legislate for same-sex marriage.
The proposals, which would see Scotland become the first part of the UK to introduce gay marriages, have provoked opposition from some religious groups
The Scottish Government is expected to announce legislation this week.
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The run is the inaugural event for the Ruth Strauss Foundation, founded by Strauss in memory of his wife who died last year.
Mr Watson blamed Labour’s ambiguity on a second referendum for a predicted loss of votes in the European elections.
Results will be declared after 10pm on Sunday.