It is highly unlikely that a massive protest would take place in major cities of Pakistan against Imran Khan’s possible arrest. Pakistan is a highly divided country politically, with different groups holding different views about Khan and the current political situation.
Additionally, the government has a heavy presence in the major cities of Pakistan, which could be used to quickly disperse any large-scale protests. Furthermore, the government has a strong track record of using force to suppress dissent, making it unlikely that such a protest would be allowed to take place.