Definition: In this task, you need to generate an appropriate title for the given summary of some paragraph. The generated title must be short and should include the main topic of the given text. Your summary should be less than 20 words long.
Input: The people gathered in Trafalgar Square stood together quietly, respectfully, flanked by police officers with the rumble of helicopters a constant presence above.

London attack: Trafalgar Square vigil for people to 'express solidarity'