Picketing
Overview
Picketing is a form of protest
Protest
A protest is an expression of objection, by words or by actions, to particular events, policies or situations. Protests can take many different forms, from individual statements to mass demonstrations...

 in which people (called picketers) congregate outside a place of work or location where an event is taking place. Often, this is done in an attempt to dissuade others from going in ("crossing the picket line
Strikebreaker
A strikebreaker is a person who works despite an ongoing strike. Strikebreakers are usually individuals who are not employed by the company prior to the trade union dispute, but rather hired prior to or during the strike to keep the organisation running...

"), but it can also be done to draw public attention to a cause. Picketers normally endeavor to be non-violent. It can have a number of aims, but is generally to put pressure on the party targeted to meet particular demands.
 
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