Computational thinking (CT) education in K-12 requires extensive teacher development. This research investigates the effectiveness of a scalable and sustainable teacher development programme in promoting CT in primary education. This report presents two studies. Study 1 reports on the effectiveness of two 12-h courses using Scratch and App Inventor programming environments for CT development of teachers respectively, which involved 245 teachers from 47 primary schools. With teaching practices embedded, the programme, which used the framework of technological pedagogical content knowledge (TPACK), significantly enhanced teachers' content-related knowledge dimensions. The results indicate that the programme helped teachers develop advanced CT concepts such as data structures and procedures. Study 2, using thematic analysis, reports on CT strategies implemented in 47 schools through 94 school visits. The most mentioned strategies were the formation of teaching teams, lesson co-planning, and the integration of CT with subject teaching. The most mentioned challenges were teacher readiness, lesson time, and diversity – learners' abilities, interests, and approaches. Four types of support, including teacher development support, continuous and communal support, teaching materials support, and financial support were identified. The results suggest that a course-based programme with mixed programming environments and embedded teaching experience effectively develops teachers’ capabilities. However, continuous support for teachers in implementing the initiative after completing the programme is important, especially in addressing learner diversity and integrating CT with subject teaching. •We evaluate a teacher development programme involving 245 teachers from 47 schools.•The programme enhanced the content-related knowledge of TPACK of these teachers.•We examine how the schools implement the CT initiative by visiting these 47 schools.•Common strategies include team formation, lesson co-planning, and subject integration.•Teacher readiness, lesson time & manpower, and learner diversity are major challenges.