"Missing \endcsname inserted" and other errors when trying to align title section

Question

I want to somehow align section headings: unnumbered should be centerized; numbered should be aligned left and indented. I'm just trying to compile an example

\documentclass[12pt,a4paper,oneside]{article}
\usepackage[left=35mm,right=15mm,top=2cm,bottom=2cm,bindingoffset=0cm]{geometry}
\usepackage[utf8x]{inputenc}
\usepackage[russian]{babel}

\usepackage{cmap}

\usepackage{makeidx}
\makeindex

\usepackage{indentfirst}
\setlength{\parindent}{1.25cm}

\usepackage{setspace}
\onehalfspacing

\usepackage{titlesec}

\titleformat{\section*}{\normalfont\Large\bfseries\justifyheading}{\thesection}{1em}{}

\begin{document}
    \section*{РЕФЕРАТ}
\end{document}

But I'm getting

! Missing \endcsname inserted.
! LaTeX Error: Missing \begin{document}.
! Extra \endcsname.
! Argument of \ttl@assign@i has an extra }.
! Paragraph ended before \ttl@assign@i was complete.
! Package keyval Error: \let \reserved@d =*\def \par .
! Missing \endcsname inserted.
! Missing number, treated as zero.
! Illegal unit of measure (pt inserted).
! Undefined control sequence.
! Missing number, treated as zero.
! Illegal unit of measure (pt inserted).
! Extra \endcsname.
! Undefined control sequence.
! Too many }'s.
! LaTeX Error: Command \CYRA unavailable in encoding OT1.

And - yes - is there a way to ONLY align sections not redefining a whole style? Should I use \newcommand instead and insert it in EVERY section?

Answer 1

There's no \justifyheading command; you can use \filcenter instead; since different formatting will be applied to numbered and unnumbered sections, you can use the options name=\section,numberless in the first argument of \titleformat for unnumbered sections, and just \section for numbered sections. The explicit option was used to have explicit access to the title and to easily achieve the required indentation for numbered sections:

\documentclass[12pt,a4paper,oneside]{article}
\usepackage[left=35mm,right=15mm,top=2cm,bottom=2cm,bindingoffset=0cm]{geometry}
\usepackage[utf8x]{inputenc}
\usepackage[russian]{babel}

\usepackage{cmap}

\usepackage{makeidx}
\makeindex

\usepackage{indentfirst}
\setlength{\parindent}{1.25cm}

\usepackage{setspace}
\onehalfspacing

\usepackage[explicit]{titlesec}

\titleformat{\section}
  {\normalfont\Large\bfseries}{}{0em}{\hspace{1.25cm}\thesection\hspace{1em}#1}% numbered sections
\titleformat{name=\section,numberless}
  {\normalfont\Large\bfseries\filcenter}{}{0em}{#1} % unnumbered sections

\usepackage{lipsum} % just to generate text

\begin{document}
    \section*{РЕФЕРАТ}
    \section{РЕФЕРАТ}
\lipsum[1-2]
\end{document}