The right to protest is one of the fundamentals of a healthy democracy, but in targeting a Labour public event in an aggressive and provocative way, the small group of protestors that screamed in Jim Murphy's face in the centre of Glasgow, led by the ubiquitous Sean Clerkin, went too far and violated that other democratic fundamental: the right to be heard.

Some passion and anger is to be expected in an election; indeed, a little more of it would be welcome in a campaign that will be remembered largely for a lack of interaction between the leaders and ordinary voters. Twenty-five years ago, John Major stood on a soapbox in the street and interacted directly with the electorate, and it was a pretty rowdy business, but in this election, there has been manic control of public appearances. A little life was shot into the campaign last week when voters were able to ask questions directly of Cameron, Clegg and Miliband on Question Time. But it has not changed the overall feel of an anodyne, bloodless few weeks in which politicians have been kept away from the people they represent.

But what happened in the centre of Glasgow was not ordinary rough and tumble. In fact, it crossed the line of what is acceptable in a healthy democracy. Mr Murphy, the comedian Eddie Izzard, and other Labour supporters were holding an event in St Enoch Square when they were confronted by Mr Clerkin and other campaigners including members of the fringe nationalist group Scottish Resistance. Attempts by Mr Murphy and others to speak were drowned out by loud music and Mr Clerkin shouting through a loudhailer. But worse than that, Mr Clerkin and his colleagues screamed in people's faces and in the pushing and shoving that followed, some in the crowd were left shaken and upset.

Mr Clerkin has plenty of experience of this kind of strategy and his aggressive targeting of the Labour leader Iain Gray during the Scottish election campaign in 2011 has even been credited by some as helping to turn the tide towards the SNP. But deliberately and aggressively shouting down an opponent so they cannot be heard is unacceptable. Eddie Izzard summed it up well when he said: "It's OK to have different opinions, but everyone should be able to put their opinion forward. We just put our view forward and then everyone makes their choice on Thursday."

In his own response to the event, Mr Murphy said aggressive nationalism should have no part in the election and it is certainly disappointing to see some nationalists using social media to justify Mr Clerkin's behaviour. However, this group of protesters belongs to a tiny minority on the Scottish political landscape and, while it is true that Mr Clerkin has a history with the SNP and once stood as a candidate for them, SNP strategists will have been cringing at the footage from St Enoch Square because they know it does nothing for their cause.

The police may also want to look again at their approach. We should be extremely wary of any attempt to force Mr Clerkin to reveal his plans to the authorities (it emerged last month that Police Scotland had asked him to do so) but equally the police have a responsibility to monitor political events to ensure politicians and their supporters can exercise the right to free speech and they do not appear to have done so in Glasgow yesterday.

As for Mr Clerkin and his supporters, they should also concentrate on the right to free speech. Instead of shouting down Mr Murphy and others, perhaps Mr Clerkin could organise his own event and spread his anti-austerity message that way. Supporters of democracy would like to think that he could do so without anyone shouting and screaming in his face and attempting to drown him out.