Shemira before the wedding:[1]
The Chasan and Kalah are not to go out alone for several days[2] prior to the wedding.[3] [Practically, many are accustomed to follow this practice one week before the wedding. Others begin to follow it the Shabbos before the wedding and others from when they arrive in the city the wedding is taking place.]

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[1] Likkutei Sichos 1/52 that so is a directive from the Rebbe Rayatz; brought in Eternal Joy 2 p.139

[2] The Rebbe Rayatz did not state an exact amount of days that is required for Shemira before the wedding. The previous Rebbe simply stated “several” days. [Likkutei Sichos ibid] Practically many are accustomed to follow this practice one week before the wedding. Others the Shabbos before the wedding and others from when they arrive in the city the wedding is taking place.

[3] The reason: The reason for this is because the time before the wedding Mazikim try to interfere with the couple. Having a guard prevents Mazikim from harming the Chasan or Kalah. This is not similar to the Shemira required after the wedding which is due to respect of the Chasan. [See Likkutei Sichos ibid]

[4] Rav Shmuel Levitin as he received from the Rebbe, recorded in Kovetz Chasuna p. 41

[5] The reason: As the reason behind this Shemira is due to Mazikin and not due to being similar to a king

[6] Aruch Hashulchan 64/3 regarding after the wedding due to the reason of Mazikin, and the same should apply before the wedding

[7] Diary of Rav Groner “The Rebbe Rayatz told the Rebbe not to be alone in a room without a Shomer, and not to pass from room to room without a Shomer”.

[8] Heard from Rav Yehuda Leib Groner