U.S. Rep. Joaquin Castro, D-San Antonio, will be outside city hall today discussing Senate Bill 4, also known as the "sanctuary cities" bill.

SB 4, signed into law on May 7 and effective Sept. 1, is a controversial law that enables law enforcement officials to inquire about the immigration status of any individual and threatens to punish cities and government officials who fail to comply with the law.

When the state law passed, Castro called it a "dark moment for Texas and for our nation."

"Not only is this law morally wrong, it will strain our law enforcement and make our communities more dangerous," he said in a statement. "Our state leaders’ fearmongering is embarrassing and harmful to all Texans."

According to a news release, Castro will join representatives of Mano Amiga, which provides educational resources to immigrants; The local chapter of the League of United Latin American Citizens, or LULAC; The Centro Cultural Hispano; The Texas State University Student Community Of Progressive Empowerment, which supports and serves undocumented students; and the San Marcos Cinema Club.

Later in the afternoon, people dressed in white are expected to congregate outside San Marcos City Hall at 4:30 p.m. before the San Marcos City Council meeting to protest the law and urge council members to join litigation against the state.

According to the release, more than 750 people have signed a petition asking City Council to participate in litigation.

The release said City Council is slated to discuss its plan for SB 4 during executive session tonight.

Several Texas cities, including the cities of Austin and San Antonio, have already filed a lawsuit challenging the constitutionality of the bill and are awaiting a ruling from a U.S. District Judge in San Antonio.

San Marcos City Council meets at 5 p.m. at San Marcos City Hall, 630 E. Hopkins St.

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Correction: This article has been updated to clarify San Marcos City Council will discuss Senate Bill 4 in executive session at Tuesday's meeting.