South Korean advocacy group activists gathered signatures of people on Saturday to prevent the "illegitimate" push to deploy a U.S. missile shield - Terminal High Altitude Area Defense (THAAD) on their territory.

At the Gwanghwamun square in central Seoul where candlelit rallies had been held to force former President Park Geun-hye out of office, the signature-collecting campaign was conducted to urge lawmakers to stop the hurried push for THAAD deployment.

On March 6, two mobile launchers and other elements of the THAAD were flown to a U.S. military base south of Seoul. One THAAD battery is composed of six mobile launchers, 48 interceptors, one X-band radar and the fire and control unit.

Local state-run broadcaster KBS reported that the AN/TPY-2 radar was supposed to be delivered to South Korea on March 16, but it was not confirmed as the U.S. Forces Korea (USFK) declined to reveal the arrival of other THAAD elements.

The hasty deployment "is illegitimate and violates the constitution," said a civic group activist who declined to be identified. She said the collected signatures would be presented next week to the parliamentary speaker and floor leaders of major political parties.

The campaign started online a week ago, and it had already collected around 5,000 signatures. They are targeting 10,000 signatures to be gathered.

The advocacy groups would call on the parliament to file for an injunction to halt all deployment procedures. The Seoul-Washington agreement to install one THAAD battery was abruptly announced in July last year without any parliamentary approval and public consensus.

"Lawmakers must stop telling. They must act," said the activist.

Separately, the activists collected signatures to ask people to join the constitutional petition, which will be filed by Lawyers for Democratic Society, an advocacy group composed of liberal lawyers, and residents of the THAAD site in southeast South Korea.

They claimed the THAAD installation lacked the residents' agreement and violated people's rights to peaceful, healthy life and environmental protection, which are guaranteed by the constitution. The petition will be filed with the constitutional court in early April after completing the signature-seeking campaign by the end of this month.

The Gwanghwamun square was packed with people holding placards that read "Opposition to THAAD." Residents thronged to the capital city and chanted the famous slogan "THAAD Out, Peace In."

Some draped a flag with anti-THAAD slogan over their shoulders, while children held a blue, rubber balloon that was stamped with anti-THAAD slogan.

In addition to opposition at home, China and Russia have strongly opposed THAAD in South Korea as it can peer deep into their territories, thus damaging security interests of the two countries and breaking regional strategic balance.

Under the Seoul-Washington agreement, THAAD in South Korea will be operated by the USFK. Seoul has claimed the X-band radar is solely aimed at the Democratic People's Republic of Korea's (DPRK) missile threats, but South Korea's military has no right to intervene in the operation.

The AN/TPY-2 radar has the terminal and the forward-based modes, which have a detecting range of around 600 km and 2,000 km respectively. Lockheed Martin, THAAD's manufacturer, is allegedly developing radar that can be changed at any time into both modes.