The sight of Jim Murphy, the Labour MP who has been travelling round Scotland making the case for a No vote on September 18, being targeted by an angry crowd was unpleasant, but does it really indicate a sinister turn in the campaign?

Mr Murphy believes so and has claimed that his speaking tour, 100 Towns in 100 Days, has fallen foul of what he calls organised street mobbery, co-ordinated, he believes, by Yes Scotland.

In response, Alex Salmond, the First Minister, has rejected Mr Murphy's concerns and has called on Scots to ignore what he calls the Labour MP's "wind-ups", although he has also made it clear that he condemns intimidation of any kind.

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A spokesman for Yes Scotland has underlined the message by saying that there is no place for abuse in the campaign and both sides of the debate should treat others with respect at all times.

Hopefully, such public statements from Yes Scotland will help maintain calm, although from the start, the independence debate has sometimes had the tendency to turn toxic and aggressive, usually on the internet. The so-called cybernats appear to be more numerous and high profile than their pro-UK counterparts but both sides have been guilty on occasion and both sides have a responsibility to control it as much as they can. There should be no hint of encouraging the nastier elements to turn on their opponents, however tempting it may be.

There is certainly nothing wrong with voters shouting or booing and making their feelings known, as they did during the recent television debates between Mr Salmond and Alistair Darling, the leader of Better Together, but when it becomes abuse or intimidation, it has to be discouraged by the leaders of both campaigns. They should always do their best to encourage their supporters to behave in a responsible and reasonable manner.

The most obvious danger of allowing the abuse to escalate, online or on the streets, is that those who would otherwise contribute to the referendum debate will give up and keep quiet rather than be shot down in flames. And that is not healthy for the wider debate.

It is this danger that is apparently worrying some in the No campaign who have expressed their concern that an overly vehement atmosphere at polling stations on September 18 could discourage some people from voting. Mr Darling is having talks with the police later this week on the matter, although the leadership of Police Scotland know a higher than normal turn-out is expected and are said to have plans in place.

It is obviously impossible to say how likely intimidation or abuse on polling day is, but passions will be running high on the day on both sides and no one should feel intimidated when they go to vote, whether they are casting their vote for the UK or for independence. Yes Scotland and Better Together must do all they can to help maintain calm at the polling stations on one of the most important days in Scotland's history.